Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosures [Table] Derivative Instruments and Hedging Activities Disclosures [Table] Hedging Designation [Axis] Hedging Designation [Axis] Hedging Designation [Domain] Hedging Designation [Domain] Hedging Relationship [Axis] Hedging Relationship [Axis] Hedging Relationship [Domain] Hedging Relationship [Domain] Derivative Instruments and Hedging Activities Disclosures [Line Items] Derivative Instruments and Hedging Activities Disclosures [Line Items] Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block] Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block] Schedule of Derivative Assets at Fair Value [Table Text Block] Schedule of Derivative Assets at Fair Value [Table Text Block] Schedule of Derivative Liabilities at Fair Value [Table Text Block] Schedule of Derivative Liabilities at Fair Value [Table Text Block] Schedule of Derivative Instruments, Gain (Loss) in Statement of Financial Performance Derivative Instruments, Gain (Loss) [Table Text Block] Accounting Policies [Abstract] Revenue Recognition, Deferred Revenue [Policy Text Block] Revenue Recognition, Deferred Revenue [Policy Text Block] Revenue Recognition, Services, Refundable Fees for Services [Policy Text Block] Revenue Recognition, Services, Refundable Fees for Services [Policy Text Block] Statement of Comprehensive Income [Abstract] Statement [Table] Statement [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] AOCI Attributable to Parent [Member] AOCI Attributable to Parent [Member] Consolidated Investment Vehicles [Member] Consolidated Investment Vehicles [Member] Voting Interest Entities and Voting Rights Entities that are consolidated into the financial statements of the registrant. The specific variable interest entities controlled by the entity may be included as children of this element. Although the registrant may not control the VIE through voting interests, it has a variable interest that will absorb a majority of the entity's expected losses, receive a majority of the entity's expected residual returns, or both. In general, a VIE is a corporation, partnership, trust, or any other legal structure used for business purposes that either (a) does not have equity investors with voting rights or (b) has equity investors that do not provide sufficient financial resources for the entity to support its activities. A VIE often holds financial assets, including loans or receivables, real estate or other property. A VIE may be essentially passive or it may engage in research and development or other activities on behalf of another company. Legal Entity [Axis] Entity [Domain] Consolidated Legg Mason, Inc. Consolidated Legg Mason [Domain] The consolidated financial information of the entity, including the portion which relates to consolidated variable interest entities. Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member] Represents amounts prior to consolidation of consolidated investment entities and eliminations. Statement [Line Items] Statement [Line Items] NET INCOME Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Other comprehensive income (loss): Other Comprehensive Income (Loss), Net of Tax [Abstract] Foreign currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, before Tax Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax Total other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax COMPREHENSIVE INCOME Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest COMPREHENSIVE INCOME ATTRIBUTABLE TO LEGG MASON, INC. Comprehensive Income (Loss), Net of Tax, Attributable to Parent Proprietary Fund Product Information [Abstract] Proprietary Fund Product Information [Abstract] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Legal Entity [Axis] Entity [Domain] Asset Class [Axis] Asset Class [Axis] Asset Class [Domain] Asset Class [Domain] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Other Comprehensive Income Location [Axis] Other Comprehensive Income Location [Axis] Other Comprehensive Income Location [Domain] Other Comprehensive Income Location [Domain] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value Measurements, Recurring and Nonrecurring [Table Text Block] Fair Value Measurements, Recurring and Nonrecurring [Table Text Block] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table Text Block] Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table Text Block] Commitments and Contingencies Disclosure [Abstract] Long-term Purchase Commitment [Table] Long-term Purchase Commitment [Table] Other Commitments [Axis] Other Commitments [Axis] Other Commitments [Domain] Other Commitments [Domain] Real Estate And Equipment [Member] Real Estate And Equipment [Member] Amount of future commitments related to real estate and equipment leases Service, Support And Maintenance Agreements [Member] Service, Support And Maintenance Agreements [Member] Amount of future commitments related to service, support and maintenance agreements Category of Item Purchased [Axis] Long-term Purchase Commitment, Category of Item Purchased [Domain] Long-term Purchase Commitment [Line Items] Operating leases Leases [Abstract] Operating Leases, Future Minimum Payments, Remainder of Fiscal Year Operating Leases, Future Minimum Payments, Remainder of Fiscal Year Operating Leases, Future Minimum Payments, Due in Two Years Operating Leases, Future Minimum Payments, Due in Two Years Operating Leases, Future Minimum Payments, Due in Three Years Operating Leases, Future Minimum Payments, Due in Three Years Operating Leases, Future Minimum Payments, Due in Four Years Operating Leases, Future Minimum Payments, Due in Four Years Operating Leases, Future Minimum Payments, Due in Five Years Operating Leases, Future Minimum Payments, Due in Five Years Operating Leases, Future Minimum Payments, Due Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Total Operating Leases, Future Minimum Payments Due Share-based Compensation [Abstract] Share-based Compensation Arrangements by Share-based Payment Award Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Title of Individual [Axis] Title of Individual [Axis] Relationship to Entity [Domain] Relationship to Entity [Domain] Key Employees [Member] Key Employees [Member] Employees that are important to the operations of Legg Mason. Consolidated Entities [Axis] Consolidated Entities [Axis] Consolidated Entities [Domain] Consolidated Entities [Domain] Subsidiaries [Member] Subsidiaries [Member] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] Employee Stock Purchase Plan [Member] Employee Stock Purchase Plan [Member] A plan where shares of the companies stock are purchased in the open market on behalf of the employees and at the option of the employee to participate in the plan. Rabbi trust [Domain] Rabbi trust [Domain] Rabbi trust Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum [Member] Maximum [Member] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Current Plan [Member] Current Plan [Member] Refers to plans that are currently in use. Employee Stock Option Employee Stock Option [Member] Restricted Stock [Member] Restricted Stock [Member] Management Equity Plan [Member] Management Equity Plan [Member] A plan that allows the key employees of an affiliate to participate in the future growth of the enterprise value subsequent to the date of grant. Share-based Compensation Arrangement by Share-based Payment Award Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Allocated Share-based Compensation Expense Allocated Share-based Compensation Expense Employee Stock Ownership Plan (ESOP), Compensation Expense Employee Stock Ownership Plan (ESOP), Compensation Expense Aggregate Cost of Awards Deferred Compensation Arrangement with Individual, Compensation Expense Variable Interest Entities and Consolidation of Investment Vehicles [Abstract] Variable Interest Entities and Consolidation of Investment Vehicles [Abstract] Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table] Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Fair Value Measured at Net Asset Value Per Share [Member] Fair Value Measured at Net Asset Value Per Share [Member] Variable Interest Entities [Axis] Variable Interest Entities [Axis] Variable Interest Entity, Classification [Domain] Variable Interest Entity, Classification [Domain] Variable Interest Entity, Primary Beneficiary [Member] Variable Interest Entity, Not Primary Beneficiary [Member] CIVs Investment Type [Axis] Investment Type [Axis] Investments [Domain] Investments [Domain] Private Equity Funds Private Equity Funds [Member] Hedge Funds Hedge Funds [Member] Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] Assets, Fair Value Disclosure Assets, Fair Value Disclosure Investment Lock up Period Investment Lock up Period The period of time that the investment is subject to a lock-up, in years. PercentageDailyRedemption PercentageDailyRedemption This element represents the percentage of the investment subject to daily redemption. Percentage of Monthly Redemption Percentage of Monthly Redemption This element represents the percentage of the investment subject to monthly redemption. Percentage of Quarterly Redemption Percentage of Quarterly Redemption This element represents the percentage of the investment subject to quarterly redemption. Percentage Subject to Lock in Period Percentage Subject to Lock in Period This element represents the percentage of the investment subject to lock-up or side pocket provisions. Alternative Investment Alternative Investment Property, Plant and Equipment, Policy [Policy Text Block] Property, Plant and Equipment, Policy [Policy Text Block] Schedule of Variable Interest Entities [Table] Schedule of Variable Interest Entities [Table] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Total Return Swap [Member] Total Return Swap [Member] Proprietary Funds [Member] Proprietary Funds [Member] Investments in Proprietary Funds Variable Interest Entity Variable Interest Entity [Line Items] Aggregate Counterparty Investment in ETF Aggregate Counterparty Investment in ETF Represents the initial amount invested into a Legg Mason sponsored ETF by a third-party investor Variable Interest Entity, Primary Beneficiary, Number of Collateralized Securites Vehicles Variable Interest Entity, Primary Beneficiary, Number of Collateralized Securites Vehicles The number of collateralized investment vehicles that are considered to be variable interest entities and for which the reporting entity is the primary beneficiary. Number of Sponsored Investment Fund Vie Number of Sponsored Investment Fund Vie Number of sponsored investment fund variable interest entities in which the reporting entity is the primary beneficiary. Number of sponsored ETF's considered VIE's Number of sponsored ETF's considered VIE's Represents the total number of sponsored ETF's considered VIE's that Legg Mason has consolidated. Employee Owned Funds Employee Owned Funds Number of funds that are seeded by employee's in which Legg Mason is the primary beneficiary. Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Director [Member] Director [Member] Incentive Distribution, Recipient [Domain] Incentive Distribution, Recipient [Domain] Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period Share-based Compensation Arrangement by Share-based Payment Award Granted Price as Percentage of Fair Market Value Minimum Share-based Compensation Arrangement by Share-based Payment Award Granted Price as Percentage of Fair Market Value Minimum Represents the minimum price as a percentage of the fair market value, at which options will be granted. Schedule Of Future Minimum Rental Payments For Operating Leases Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block] Lease Liability Reserve Roll Forward [Table Text Block] Other Noncurrent Liabilities [Table Text Block] Schedule of Business Acquisitions by Acquisition, Contingent Consideration [Table Text Block] Schedule of Business Acquisitions by Acquisition, Contingent Consideration [Table Text Block] Revenue from Contract with Customer [Abstract] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Product and Service [Axis] Product and Service [Axis] Product and Service [Domain] Product and Service [Domain] Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] UNITED STATES UNITED STATES UNITED KINGDOM UNITED KINGDOM Non-US [Member] Non-US [Member] Contract with Customer, Basis of Pricing [Axis] Contract with Customer, Basis of Pricing [Axis] Contract with Customer, Basis of Pricing [Domain] Contract with Customer, Basis of Pricing [Domain] Contract with Customer, Duration [Axis] Contract with Customer, Duration [Axis] Contract with Customer, Duration [Domain] Contract with Customer, Duration [Domain] Timing of Transfer of Good or Service [Axis] Timing of Transfer of Good or Service [Axis] Timing of Transfer of Good or Service [Domain] Timing of Transfer of Good or Service [Domain] Contract with Customer, Sales Channel [Axis] Contract with Customer, Sales Channel [Axis] Contract with Customer, Sales Channel [Domain] Contract with Customer, Sales Channel [Domain] Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Total Operating Revenues Revenues Retirement Benefits [Abstract] Schedule of Defined Benefit Plans Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Martin Currie [Member] Martin Currie [Member] An affiliate, Martin Currie, that is a leading international equity specialist based in Edinburgh, Scotland. Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Retirement Plan Sponsor Location [Axis] Retirement Plan Sponsor Location [Axis] Retirement Plan Sponsor Location [Domain] Retirement Plan Sponsor Location [Domain] Retirement Plan Tax Status [Axis] Retirement Plan Tax Status [Axis] Retirement Plan Tax Status [Domain] Retirement Plan Tax Status [Domain] Retirement Plan Funding Status [Axis] Retirement Plan Funding Status [Axis] Retirement Plan Funding Status [Domain] Retirement Plan Funding Status [Domain] Retirement Plan Name [Axis] Retirement Plan Name [Axis] Retirement Plan Name [Domain] Retirement Plan Name [Domain] Defined Benefit Plan, Plan Assets, Category [Axis] Defined Benefit Plan, Plan Assets, Category [Axis] Defined Benefit Plan, Plan Assets, Category [Domain] Defined Benefit Plan, Plan Assets, Category [Domain] Industry Sector [Axis] Industry Sector [Axis] Industry Sector [Domain] Industry Sector [Domain] Investment Objective [Axis] Investment Objective [Axis] Investment Objective [Domain] Investment Objective [Domain] Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] Related Party Transaction [Axis] Related Party Transaction [Axis] Related Party Transaction [Domain] Related Party Transaction [Domain] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay Defined Contribution Plan, Employer Discretionary Contribution Amount Defined Contribution Plan, Employer Discretionary Contribution Amount Defined Contribution Plan, Employer Matching Contribution, Percent of Match Defined Contribution Plan, Employer Matching Contribution, Percent of Match Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year Underlying Asset Class [Axis] Underlying Asset Class [Axis] Underlying Asset Class [Domain] Underlying Asset Class [Domain] Derivative Financial Instruments, Assets [Member] Derivative Financial Instruments, Assets [Member] Future [Member] Interest Rate Swap [Member] Interest Rate Swap [Member] Variable Interest Entity, Primary Beneficiary [Member] Variable Interest Entity, Primary Beneficiary [Member] Foreign Mutual Funds [Member] Foreign Mutual Funds [Member] Mutual Funds that are domiciled outside of the Untied States Sponsored Investment Fund [Member] Sponsored Investment Fund [Member] Funds in which Legg Mason is the sponsor and primary beneficiary Employee Owned Funds [Member] Employee Owned Funds [Member] Funds in which Legg Mason (or Legg Mason Affiliate) employees are the sponsors of the fund and have investments, but Legg Mason is the primary beneficiary. Sponsored ETF [Member] Sponsored ETF [Member] ETF's that Legg Mason sponsors and is the primary beneficiary NumberOfDeconsolidatedFunds NumberOfDeconsolidatedFunds Number of Funds Deconsolidated Under New Accounting Guidance Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount Derivative, Notional Amount Derivative, Notional Amount Number of Funds Number of Funds The number of funds that Legg Mason consolidates as VIE's, by type of fund. Property, Plant and Equipment [Abstract] Fixed Assets Property, Plant and Equipment Disclosure [Text Block] Consolidation, Policy [Policy Text Block] Consolidation, Policy [Policy Text Block] Revenue Recognition, Policy [Policy Text Block] Revenue Recognition, Policy [Policy Text Block] Cash and Cash Equivalents, Restricted Cash and Cash Equivalents, Policy [Policy Text Block] Cash and Cash Equivalents, Restricted Cash and Cash Equivalents, Policy [Policy Text Block] Investment, Policy [Policy Text Block] Investment, Policy [Policy Text Block] New Accounting Pronouncements, Policy [Policy Text Block] New Accounting Pronouncements, Policy [Policy Text Block] Revenue, Initial Application Period Cumulative Effect Transition [Table] Revenue, Initial Application Period Cumulative Effect Transition [Table] Capitalized Contract Cost [Axis] Capitalized Contract Cost [Axis] Capitalized Contract Cost [Domain] Capitalized Contract Cost [Domain] Long-term Contract with Customer [Member] Long-term Contract with Customer [Member] Contingent Deferred Sales Charges [Member] Contingent Deferred Sales Charges [Member] Contingent Deferred Sales Charges [Member] Nature of Expense [Axis] Nature of Expense [Axis] Interim Period, Costs Not Allocable [Domain] Interim Period, Costs Not Allocable [Domain] SMA Commissions [Member] SMA Commissions [Member] SMA Commissions [Member] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] RETAINED EARNINGS Retained Earnings [Member] Other Current Assets [Member] Other Current Assets [Member] Other Noncurrent Assets [Member] Other Noncurrent Assets [Member] Deferred Tax Asset [Domain] Deferred Tax Asset [Domain] Deferred Tax Liability [Member] Deferred Tax Liability [Member] Deferred Tax Liability [Member] Initial Application Period Cumulative Effect Transition [Axis] Initial Application Period Cumulative Effect Transition [Axis] Initial Application Period Cumulative Effect Transition [Domain] Initial Application Period Cumulative Effect Transition [Domain] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Revenue from Contract with Customer [Member] Revenue from Contract with Customer [Member] Revenue, Initial Application Period Cumulative Effect Transition [Line Items] Revenue, Initial Application Period Cumulative Effect Transition [Line Items] New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification Other current assets Other Assets, Current Cumulative Effect of New Accounting Principle in Period of Adoption Cumulative Effect of New Accounting Principle in Period of Adoption Deferred income taxes Deferred Tax Assets, Net of Valuation Allowance, Noncurrent Other non-current assets Other Assets, Noncurrent Deferred income taxes Deferred Tax Liabilities, Net, Noncurrent Retained earnings Retained Earnings, Unappropriated Deferred Sales Commission Deferred Sales Commission Debt Disclosure [Abstract] Schedule of Long-term Debt Instruments [Table Text Block] Schedule of Long-term Debt Instruments [Table Text Block] Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Property, Plant and Equipment by Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment Property, Plant and Equipment [Line Items] Depreciation and amortization Depreciation, Depletion and Amortization Accelerated Depreciation, Other Accelerated Depreciation, Other Amount of accelerated depreciation charged against earnings that is for purposes other than restructuring. Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Income Statement Location [Axis] Income Statement Location [Domain] Other Nonoperating income (expense) [Member] Other Nonoperating Income (Expense) [Member] Other Liabilities [Member] Other Liabilities [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Derivative liabilities Derivative Liability Gain on Derivative Assets and Liabilities Recorded in Other Non-Operating Income of Consolidated Investment Vehicles Derivative, Gain on Derivative Loss on Derivative Assets and Liabilities Recorded in Other Non-Operating Income of Consolidated Investment Vehicles Derivative, Loss on Derivative Goodwill and Intangible Assets, Policy [Policy Text Block] Goodwill and Intangible Assets, Policy [Policy Text Block] Management and Investment Advisory Fees, Policy [Policy Text Block] Management and Investment Advisory Fees, Policy [Policy Text Block] Long-term Debt, by Current and Noncurrent [Abstract] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] 2.7% Senior Notes [Member] 2.7% Senior Notes [Member] 2.7% Senior Notes Due July 2019 [Member] 3.95% Senior Notes [Member] 3.95% Senior Notes [Member] 3.95% Senior Notes due July 2024 [Member] 4.75% Senior Notes [Member] 4.75% Senior Notes [Member] 4.75% Senior Notes [Member] 5.45% Junior Subordinated Notes [Member] 5.45% Junior Subordinated Notes [Member] 5.45% Junior Subordinated Notes issued in August 2016 due in August 2056 [Member] 5.625% Senior Notes [Member] 5.625% Senior Notes [Member] [Member] 5.625% Senior Notes [Member] 6.375% Junior Notes [Member] 6.375% Junior Notes [Member] 6.375% Junior Notes [Member] Long-term Debt, Gross Long-term Debt, Gross Long-term Debt Long-term Debt Debt Instrument, Fair Value Disclosure Debt Instrument, Fair Value Disclosure Debt Instrument, Unamortized Discount (Premium), Net Debt Instrument, Unamortized Discount (Premium), Net Debt Instrument, Unamortized Premium Debt Instrument, Unamortized Premium Debt Instrument, Unamortized Discount Debt Instrument, Unamortized Discount Unamortized Debt Issuance Expense Unamortized Debt Issuance Expense Long-term Debt, Maturities, Repayments of Principal in Year Three Long-term Debt, Maturities, Repayments of Principal in Year Three Long-term Debt, Maturities, Repayments of Principal after Year Five Long-term Debt, Maturities, Repayments of Principal after Year Five Long-term Debt, Fair Value Long-term Debt, Fair Value Earnings Per Share [Abstract] Earnings Per Share [Text Block] Earnings Per Share [Text Block] Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Weighted Average Remaining Contractual Terms Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Weighted Average Remaining Contractual Terms Text Block [Abstract] Investment Income [Table Text Block] Investment Income [Table Text Block] Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Finite- Lived and Indefinite-lived Intangible Assets by Major Class [Table] Schedule of Finite- Lived and Indefinite-lived Intangible Assets by Major Class [Table] In the period of acquisition, an Entity must disclose disclosure of the details of intangible assets not subject to amortization, excluding goodwill, in total and by major class and the characteristics of finite-lived intangible assets acquired. Weighted Average [Member] Weighted Average [Member] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] EnTrust Capital [Member] EnTrust Capital [Member] Newly acquired affiliate that will immediately become EnTrustPermal. RARE Infrastructure, Ltd [Member] RARE Infrastructure, Ltd [Member] Related to information related specifically to our affiliate, RARE Infrastructure QS Investors [Domain] QS Investors [Domain] QS Investors [Domain] Clarion Partners [Member] Clarion Partners [Member] Clarion Partners is a newly acquired company that will be a stand alone affiliate once integrated into Legg Mason. EnTrustPermal [Member] EnTrustPermal [Member] New affiliate created from the acquisition of EnTrust and merging Permal into it to create a newly branded affiliate, EnTrustPermal. Permal [Member] Permal [Member] Represents the acquired business of Permal. Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Asset Management Contracts Assets management contracts [Member] Represents amortizable asset management contracts member. Trade Names Trade Names [Member] Indefinite-lived Intangible Assets [Axis] Indefinite-lived Intangible Assets [Axis] Indefinite-lived Intangible Assets, Major Class Name [Domain] Indefinite-lived Intangible Assets, Major Class Name [Domain] U.S. Domestic Mutual Fund Management Contracts U.S. domestic mutual fund management contracts [Member] Related to the U.S. domestic mutual fund management contracts Other Fund Management Contracts [Member] Other Fund Management Contracts [Member] Other Fund Management Contracts [Member] Finite-Lived and Indefinite-lived Intangible Assets Finite-Lived and Indefinite-Lived Intangible Assets [Line Items] Finite-Lived and Indefinite-Lived Intangible Assets [Line Items] Cost Finite-Lived Intangible Assets, Gross Accumulated amortization Finite-Lived Intangible Assets, Accumulated Amortization Total Finite-Lived Intangible Assets, Net Indefinite-life intangible assets Indefinite-lived Intangible Assets (Excluding Goodwill) Intangible assets, net Intangible Assets, Net (Excluding Goodwill) Organization, Consolidation and Presentation of Financial Statements [Abstract] Schedule of Finite-Lived Intangible Assets Future Amortization Expense [Table] This element represents the disclosure of estimated amortization expense of intangible assets, for each of the five succeeding fiscal years. Fair Value, Inputs, Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Customer Contracts [Member] Customer Contracts [Member] Finite-Lived Intangible Assets Future Amortization Expense [Line Items] Finite-Lived Intangible Assets Future Amortization Expense [Line Items] Finite-Lived Intangible Assets Future Amortization Expense [Line Items] Change in Assets Under Management Change in Assets Under Management Change in Assets Under Management Impairment of Intangible Assets Asset Impairment Charges Finite-Lived Intangible Assets, Amortization Expense, Remainder of Fiscal Year Finite-Lived Intangible Assets, Amortization Expense, Remainder of Fiscal Year Finite-Lived Intangible Assets, Amortization Expense, Year Two Finite-Lived Intangible Assets, Amortization Expense, Year Two Finite-Lived Intangible Assets, Amortization Expense, Year Three Finite-Lived Intangible Assets, Amortization Expense, Year Three Finite-Lived Intangible Assets, Amortization Expense, Year Four Finite-Lived Intangible Assets, Amortization Expense, Year Four Finite-Lived Intangible Assets, Amortization Expense, Year Five Finite-Lived Intangible Assets, Amortization Expense, Year Five Thereafter Finite-Lived Intangible Assets, Amortization Expense, after Year Five Finite-Lived Intangible Asset, Useful Life Finite-Lived Intangible Asset, Useful Life Fair Value Inputs, Long-term AUM Growth Rate Fair Value Inputs, Long-term AUM Growth Rate Percentage of assumed long-term growth in AUM, used as an input to measure fair value. Fair Value Inputs, Attrition Rate Fair Value Inputs, Attrition Rate Amount used to represent the attrition when calculating the fair value of amortizable asset management contracts Fair Value Input, Projected Cash Flow Growth Rate, Average Fair Value Input, Projected Cash Flow Growth Rate, Average Percentage of assumed growth rate in projected cash flows, used as an input to measure fair value, average Fair Value Input, Discount Rate Fair Value Input, Discount Rate Discount rate used to determine fair value measurement Statement of Stockholders' Equity [Abstract] Royce [Member] Royce [Member] Royce is a wholly owned affiliate of Legg Mason, that invests primarily in Small Cap Companies. COMMON STOCK Common Stock [Member] ADDITIONAL PAID-IN CAPITAL Additional Paid-in Capital [Member] EMPLOYEE STOCK TRUST Employee Stock Trust [Member] Common stock issued to a trust (for example, a 'rabbi trust') set up specifically to accumulate stock for the sole purpose of distribution to participating employees. This trust does not allow employees to immediately or after a holding period diversify into nonemployer securities. The deferred compensation plan for which this trust is set up must be settled by the delivery of a fixed number of shares of employer stock. DEFERRED COMPENSATION EMPLOYEE STOCK TRUST Deferred Compensation Employee Stock Trust [Member] Common stock issued to a deferred compensation employee stock trust, which is set up specifically to accumulate stock for the sole purpose of distribution to participating employees. This trust does not allow employees to immediately or after a holding period diversify into nonemployer securities. The deferred compensation plan for which this trust is set up must be settled by the delivery of a fixed number of shares of employer stock. ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS), NET Stockholders' Equity Attributable to Parent Stockholders' Equity Attributable to Parent Stockholders' Equity Attributable to Noncontrolling Interest Stockholders' Equity Attributable to Noncontrolling Interest Net Income (Loss) Attributable to Nonredeemable Noncontrolling Interest Net Income (Loss) Attributable to Nonredeemable Noncontrolling Interest Deferred Compensation Arrangement with Individual, Allocated Share-based Compensation Expense Deferred Compensation Arrangement with Individual, Allocated Share-based Compensation Expense Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Beginning Balance Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Stock options exercised Adjustments to Additional Paid in Capital, Share-based Compensation and Exercise of Stock Options Deferred compensation employee stock trust Deferred Compensation Employee Stock Trust, Stockholders Equity Impact Aggregate change in value for phantom 401k transactions during the period. Stock-based compensation Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures Adjustments to Additional Paid in Capital, Other Adjustments to Additional Paid in Capital, Other Employee tax withholdings by settlement of net share transactions Adjustments Related to Tax Withholding for Share-based Compensation Shares repurchased and retired Stock Repurchased and Retired During Period, Value APIC reclassified for MEP vesting APIC reclassified for MEP vesting Additional paid-in capital reclassified for management equity plan vesting Allocation from Convertible Senior Notes Net of Tax Allocation from Convertible Senior Notes Net of Tax Allocation from the repurchase of the 2.5% Convertible Senior Notes, Net of Tax, as displayed in the statement of changes in stockholders' equity Shares Issued to plans Shares Issued to Employee Stock Trust Plans Aggregate change in value for stock issued during the period as a result of employee stock trust plans. Distributions and forfeitures Distributions and Forfeitures to Employee Stock Trust Plans Value of stock related to Employee Stock Trust Plans that was distributed and forfeited during the period. Net Income Attributable to Legg Mason, Inc. Net Income (Loss) Attributable to Parent Dividends declared Dividends Reclassification from Retained Earnings to Noncontrolling Interests Reclassification from Retained Earnings to Noncontrolling Interests The proper accounting for some of our equity arrangements with affiliates as well as other activity requires a reclassification from retained earnings into noncontrolling interests in order to appropriately maintain the fair value of or record accretion related to noncontrolling interests RE Reclassified for MEP Amortization RE Reclassified for MEP Amortization This is the accretion on the MEP plans and it is reclassified out of retained earnings and into Noncontrolling Interests throughout the quarter Excess Tax Benefit from Share-based Compensation, Financing Activities Excess Tax Benefit from Share-based Compensation, Financing Activities Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax Ending Balance Stock Issued During Period, Value, Stock Options Exercised Stock Issued During Period, Value, Stock Options Exercised Common Stock Issued, Employee Stock Trust Common Stock Issued, Employee Stock Trust Employee Tax Withholdings Related to net share transaction Employee Tax Withholdings Related to net share transaction Employee Tax Withholdings Related to net share transactions Stockholders' Equity Attributable to Parent Stockholders' Equity Attributable to Noncontrolling Interest Noncontrolling Interest [Abstract] Noncontrolling Interest Disclosure [Text Block] Noncontrolling Interest Disclosure [Text Block] Contingent Liability Reserve Estimate, Policy [Policy Text Block] Contingent Liability Reserve Estimate, Policy [Policy Text Block] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Basis [Axis] Measurement Basis [Axis] Portion at Fair Value Measurement [Member] Portion at Fair Value Measurement [Member] Fair Value, Inputs, Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Condensed Income Statement [Table] Condensed Income Statement [Table] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Consolidation, Eliminations [Member] Consolidation, Eliminations [Member] Consolidated Entity Excluding Variable Interest Entities (VIE) [Member] Consolidated Entity Excluding Variable Interest Entities (VIE) [Member] Condensed Income Statements, Captions [Line Items] Condensed Income Statements, Captions [Line Items] Total Operating Expenses Operating Expenses OPERATING INCOME Operating Income (Loss) Total Other Non-Operating Income (Expense) Nonoperating Income (Expense) INCOME BEFORE INCOME TAX PROVISION Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Income tax provision Income Tax Expense (Benefit) Net Income (Loss) Attributable to Noncontrolling Interest Net Income (Loss) Attributable to Noncontrolling Interest Stock-Based Compensation Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Investments, Debt and Equity Securities [Abstract] Net Realized and Unrealized Gain (Loss) on Trading Securities Net Realized and Unrealized Gain (Loss) on Trading Securities Trading Securities, Unrealized Holding Gain Trading Securities, Change in Unrealized Holding Gain (Loss) Debt, Policy [Policy Text Block] Debt, Policy [Policy Text Block] Revenue from Contract with Customer [Text Block] Revenue from Contract with Customer [Text Block] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Legal Entity of Counterparty, Type [Axis] Legal Entity of Counterparty, Type [Axis] Legal Entity Type of Counterparty [Domain] Legal Entity Type of Counterparty [Domain] Shanda Asset Management Investment Limited [Member] Shanda Asset Management Investment Limited [Member] Represents transactions that happened with Shanda Asset Management Investment Limited Class of Stock [Line Items] Class of Stock [Line Items] Stock Repurchased and Retired During Period, Shares Stock Repurchased and Retired During Period, Shares Stock Repurchased and Retired During Period, Value Stock Repurchase Program, Remaining Authorized Repurchase Amount Stock Repurchase Program, Remaining Authorized Repurchase Amount Stock Repurchase Program, Authorized Amount Stock Repurchase Program, Authorized Amount Shares Paid for Tax Withholding for Share Based Compensation Shares Paid for Tax Withholding for Share Based Compensation Adjustments Related to Tax Withholding for Share-based Compensation Weighted Average Number Diluted Shares Outstanding Adjustment Weighted Average Number Diluted Shares Outstanding Adjustment Stock Repurchased During Period, Shares Stock Repurchased During Period, Shares Repurchase of common stock Payments for Repurchase of Common Stock Noncontrolling Interest [Table] Noncontrolling Interest [Table] CIVs and Other Noncontrolling Interest [Member] Noncontrolling Interest [Member] Noncontrolling Interest [Line Items] Noncontrolling Interest [Line Items] Redeemable Noncontrolling Interest, Equity, Carrying Amount Redeemable Noncontrolling Interest, Equity, Carrying Amount Net Income (Loss) Attributable to Redeemable Noncontrolling Interest Net Income (Loss) Attributable to Redeemable Noncontrolling Interest Proceeds from (Payments to) Noncontrolling Interests Proceeds from (Payments to) Noncontrolling Interests APIC reclassified for MEP vesting Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value Noncontrolling Interest, Period Increase (Decrease), Foreign Exchange Noncontrolling Interest, Period Increase (Decrease), Foreign Exchange The impact of foreign exchange on the balance of noncontrolling interest holders RE Reclassified for MEP Amortization Noncontrolling Interest, Change in Redemption Value Noncontrolling Interest, Change in Redemption Value Stockholders' Equity Attributable to Noncontrolling Interest Statement of Financial Position [Abstract] Currency [Axis] Currency [Axis] All Currencies [Domain] All Currencies [Domain] United Kingdom, Pounds United Kingdom, Pounds Fair Value Adjustment, by Type [Axis] Fair Value Adjustment, by Type [Axis] Will be used to distinguish between the different reasons for the change in the fair value of an asset or liability. Examples include, foreign exchange, Change in Assumptions, etc. Fair Value Adjustment, by Type [Domain] Fair Value Adjustment, by Type [Domain] [Domain] for Will be used to distinguish between the different reasons for the change in the fair value of an asset or liability. Examples include, foreign exchange, Change in Assumptions, etc. Change in Input Assumptions [Member] Change in Input Assumptions [Member] Represents changes in the fair value of an asset or liability resulting from changes in the inputs used to determine the fair value of the asset or liability. Other changes in Fair Value [Member] Other changes in Fair Value [Member] Represents changes in the fair value of assets or liabilities caused by changes in exchange rates, accretion and other miscellaneous adjustments Vesting [Axis] Vesting [Axis] Vesting [Domain] Vesting [Domain] Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] Other Segments [Member] Other Segments [Member] Trade Names [Member] Permal funds-of-hedge fund management contracts [Member] Permal funds-of-hedge fund management contracts [Member] Related to the Permal funds-of-hedge fund management contracts Financial Guard LLC [Member] Financial Guard LLC [Member] Financial Guard LLC is an online registered investment advisor and innovative technology-enabled wealth management and investment advice platform company acquired by Legg Mason Inc [Member] Employee Stock Option [Member] Long Term Incentive Compensation Plans [Member] Long Term Incentive Compensation Plans [Member] Represents the fair value of long term incentive compensation plans recorded as trading investments ClearBridge Investments [Member] ClearBridge Investments [Member] ClearBridge Investments [Member] Deferred Compensation Arrangement with Individual, Fair Value of Shares Issued Deferred Compensation Arrangement with Individual, Fair Value of Shares Issued ASSETS Assets [Abstract] Current Assets Assets, Current [Abstract] Cash and Cash Equivalents Cash and Cash Equivalents, at Carrying Value Restricted Cash and Cash Equivalents, Current Restricted Cash and Cash Equivalents, Current Receivables: Receivables, Net, Current [Abstract] Investment advisory and related fees Accounts Receivable, Net, Current Other receivables Other Receivables, Net, Current Investment securities Short-term Investments Total Current Assets Assets, Current Fixed assets, net Property, Plant and Equipment, Net Goodwill Goodwill TOTAL ASSETS Assets LIABILITIES AND STOCKHOLDERS' EQUITY Liabilities and Equity [Abstract] LIABILITIES Liabilities [Abstract] Current Liabilities Liabilities, Current [Abstract] Accrued compensation Employee-related Liabilities, Current Accounts Payable and Accrued Liabilities, Current Accounts Payable and Accrued Liabilities, Current Line of Credit, Current Line of Credit, Current Business Combination, Contingent Consideration, Liability, Current Business Combination, Contingent Consideration, Liability, Current Other current liabilities Other Liabilities, Current Total Current Liabilities Liabilities, Current Deferred compensation Deferred Compensation Liability, Classified, Noncurrent Business Combination, Contingent Consideration, Liability, Noncurrent Business Combination, Contingent Consideration, Liability, Noncurrent Other non-current liabilities Other Liabilities, Noncurrent Long-term Debt Long-term Debt, Excluding Current Maturities TOTAL LIABILITIES Liabilities REDEEMABLE NONCONTROLLING INTERESTS STOCKHOLDERS' EQUITY Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Common stock, par value $.10; authorized 500,000,000 shares; issued 85,440,021 shares for June 2018 and 84,606,408 shares in March 2017 Common Stock, Value, Issued Additional paid-in capital Additional Paid in Capital Employee stock trust Deferred compensation employee stock trust Compensation and Benefits Trust AOCI Including Portion Attributable to Noncontrolling Interest, before Tax AOCI Including Portion Attributable to Noncontrolling Interest, before Tax TOTAL STOCKHOLDERS' EQUITY Stockholders' Equity Attributable to Parent TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY Liabilities and Equity Common Stock, Par or Stated Value Per Share Common Stock, Par or Stated Value Per Share Common Stock, Shares Authorized Common Stock, Shares Authorized Common Stock, Shares, Issued Common Stock, Shares, Issued Statement of Cash Flows [Abstract] Income Tax Authority [Axis] Income Tax Authority [Axis] Income Tax Authority [Domain] Income Tax Authority [Domain] Domestic Tax Authority [Member] Domestic Tax Authority [Member] 5.5% Senior Notes [Member] 5.5% Senior Notes [Member] 5.5% Senior Notes Restricted Cash and Cash Equivalents, Noncurrent Restricted Cash and Cash Equivalents, Noncurrent Income Taxes Paid, Net Income Taxes Paid, Net CASH FLOWS FROM OPERATING ACTIVITIES Net Cash Provided by (Used in) Operating Activities [Abstract] Net Income Payment for Debt Extinguishment or Debt Prepayment Cost Payment for Debt Extinguishment or Debt Prepayment Cost Adjustments to reconcile Net Income to net cash used in operations: Adjustments, Noncash Items, to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill) Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill) Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount Depreciation and amortization Depreciation and Amortization Excluding Amortization of Deferred Sales Commissions The aggregate amount of depreciation expense and amortization expense. Depreciation expense is the noncash expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Amortization expense is the noncash expense charged against earnings in the current period that reflects the allocation of the cost of definite lived intangible assets and leasehold improvements over the shorter of their estimated remaining economic lives or the lease term. Excludes the amortization of deferred sales commissions. Accretion and amortization of securities discounts and premiums, net Accretion (Amortization) of Discounts and Premiums, Investments Stock-based compensation Share-based Compensation Unrealized Gain (Loss) on Investments Unrealized Gain (Loss) on Investments Net (gains) losses on investments Gain (Loss) on Investments Deferred income taxes Deferred Income Tax Expense (Benefit) Contingent consideration fair value adjustments Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability Other Other Noncash Income (Expense) Decrease (increase) in assets: Increase (Decrease) in Operating Assets [Abstract] Investment advisory and related fees receivable Increase (Decrease) in Accounts Receivable Net (purchases) sales of trading and other current investments Proceeds from (Payments for) Trading Securities, Short-term Other receivables Increase (Decrease) in Other Receivables Other assets Increase (Decrease) in Other Operating Assets Increase (decrease) in liabilities: Increase (Decrease) in Operating Liabilities [Abstract] Accrued compensation Increase (Decrease) in Employee Related Liabilities Deferred compensation Increase (Decrease) in Deferred Compensation Accounts payable and accrued expenses Increase (Decrease) in Accounts Payable and Accrued Liabilities Other liabilities Increase (Decrease) in Other Operating Liabilities CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES Net Cash Provided by (Used in) Operating Activities, Continuing Operations CASH FLOWS FROM INVESTING ACTIVITIES Net Cash Provided by (Used in) Investing Activities [Abstract] Payments for fixed assets Payments to Acquire Property, Plant, and Equipment Business acquisitions, net of cash acquired of $29,830 Payments to Acquire Businesses, Net of Cash Acquired Payments to Acquire Assets, Investing Activities Payments to Acquire Assets, Investing Activities Proceeds from Sales of Assets, Investing Activities Proceeds from Sales of Assets, Investing Activities Contingent Payment Received, Investing Activities Contingent Payment Received, Investing Activities A payment received in connection with a prior sale of a business based on meeting certain contingencies Cash Acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents Change in restricted cash Increase (Decrease) in Restricted Cash Proceeds from sales and maturities of investment securities Proceeds from Sale and Maturity of Marketable Securities CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES Net Cash Provided by (Used in) Investing Activities CASH FLOWS FROM FINANCING ACTIVITIES Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from Lines of Credit Proceeds from Lines of Credit Debt issuance costs Payments of Debt Issuance Costs Payment for Contingent Consideration Liability, Financing Activities Payment for Contingent Consideration Liability, Financing Activities Business Acquisition, Preacquisition Contingency, Amount of Settlement Payment for Contingent Consideration Liability, Operating Activities Issuance of common stock for stock-based compensation Proceeds from Issuance of Common Stock Proceeds from Issuance of Long-term Debt Proceeds from Issuance of Long-term Debt Dividends paid Payments of Dividends CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES Net Cash Provided by (Used in) Financing Activities Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Interest Paid, Including Capitalized Interest, Operating and Investing Activities Interest Paid, Including Capitalized Interest, Operating and Investing Activities Proceeds from Income Tax Refunds Proceeds from Income Tax Refunds Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Fair Value Disclosures [Abstract] Fair Values of Assets and Liabilities Fair Value Disclosures [Text Block] Income Statement [Abstract] Separate Accounts [Member] Separate Accounts [Member] A member of the Investment Type axis to segregate out the portion of assets, revenue, etc that is related to Separate Accounts Mutual Funds [Member] Mutual Funds [Member] This is a member of the Investment Type axis and is used to segregate out the portion of assets, revenues, etc related to Mutuals Funds Performance Fees [Member] Performance Fees [Member] Market Input used to calculate the fair value of the contingent consideration. Distribution and Shareholder Service [Member] Distribution and Shareholder Service [Member] Restructuring Charges [Member] Restructuring Charges [Member] OPERATING REVENUES Revenues [Abstract] Revenue from Contract with Customer, Excluding Assessed Tax Revenue from Contract with Customer, Excluding Assessed Tax Other Income Other Income OPERATING EXPENSES Costs and Expenses [Abstract] Compensation and benefits Labor and Related Expense Distribution and servicing Noninterest Expense Distribution and Servicing Fees Communications and technology Communications and Information Technology Occupancy Occupancy, Net Amortization of intangible assets Amortization of Intangible Assets Other Other Expenses Total Operating Expenses OTHER NON-OPERATING INCOME (EXPENSE) Nonoperating Income (Expense) [Abstract] Interest income Investment Income, Interest Interest expense Interest Expense Other income (expense), net Other Nonoperating Income (Expense) INCOME BEFORE INCOME TAX PROVISION NET INCOME NET INCOME ATTRIBUTABLE TO LEGG MASON, INC. NET INCOME PER SHARE ATTRIBUTABLE TO LEGG MASON, INC. COMMON SHAREHOLDERS Basic (in dollars per share) Earnings Per Share, Basic Diluted (in dollars per share) Earnings Per Share, Diluted Common Stock, Dividends, Per Share, Declared Common Stock, Dividends, Per Share, Declared Components of fixed assets Property, Plant and Equipment [Table Text Block] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Funds-Of-Hedge Funds [Member] Funds-of-hedge funds [Member] Represents the funds-of-hedge funds category of investment. Real Estate Funds [Member] Real Estate Funds [Member] Other Investments [Member] Other Security Investments [Member] Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments Percentage Not Subject to Redemption Percentage Not Subject to Redemption This element represents the percentage of the investment which is not subject to redemption restrictions. Percentage of Redemption, Investment Funds Percentage of Redemption, Investment Funds This element represents the percentage of the investment funds that are subject to redemption (which may be monthly, quarterly, or annually) and have a remaining term of less than one year. Percentage of Investment of 20 Year Redemption Percentage of Investment of 20 Year Redemption This element represents the percentage of investment having a 20 year redemption period. Investments Remaining Term, High End of Range Investments Remaining Term, High End of Range Investments Remaining Term, High End of Range Investments Remaining Term, Category One Investments Remaining Term, Category One Investments Remaining Term, Category One Investments Remaining Term, Category Two Investments Remaining Term, Category Two Investments Remaining Term, Category Two Pension and Other Postretirement Benefits Disclosure [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] Compensation and Employee Benefit Plans [Text Block] Compensation and Employee Benefit Plans [Text Block] Derivative [Table] Derivative [Table] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Revolving Credit Facility [Member] Revolving Credit Facility [Member] Not Designated as Hedging Instrument [Member] Not Designated as Hedging Instrument [Member] Foreign Exchange Forward [Member] Foreign Exchange Forward [Member] Future [Member] Future [Member] Total Return Swap Expiring in May 2019 [Member] Total Return Swap Expiring in May 2019 [Member] Total Return Swap Expiring in May 2019 [Member] Total Return Swap Expiring in May 2019_2 [Member] Total Return Swap Expiring in May 2019_2 [Member] Total Return Swap Expiring in May 2019_2 [Member] Total Return Swap Expiring in April 2019 [Member] Total Return Swap Expiring in April 2019 [Member] Total Return Swap Expiring in April 2019 [Member] Total Return Swap Expiring in July 2018 [Member] Total Return Swap Expiring in July 2018 [Member] Used to identify information related to a Total Return Swap Expiring in July 2018 [Member] Other Assets [Member] Other Assets [Member] Derivative Derivative [Line Items] Amount Net for Fair Value of Derivative Assets and (Liabilities) Derivative Assets (Liabilities), at Fair Value, Net Derivative, Term of Contract Derivative, Term of Contract Derivative, Fixed Interest Rate Derivative, Basis Spread on Variable Rate Derivative, Average Remaining Maturity Derivative, Average Remaining Maturity Line of Credit Facility [Table] Line of Credit Facility [Table] Line of Credit Facility [Line Items] Line of Credit Facility [Line Items] Line of Credit Facility, Annual Principal Payment Line of Credit Facility, Annual Principal Payment Line of Credit Facility, Increase (Decrease), Net Line of Credit Facility, Increase (Decrease), Net Line of Credit Facility, Fair Value of Amount Outstanding Line of Credit Facility, Fair Value of Amount Outstanding Line of Credit Facility, Maximum Borrowing Capacity Line of Credit Facility, Maximum Borrowing Capacity Line of Credit Facility, Additional Capacity Available Line of Credit Facility, Additional Capacity Available Amount of capacity that is available to increase the maximum borrowing capacity under the facility. Debt Covenant, Cash Collateral Provision Debt Covenant, Cash Collateral Provision The maximum amount of liens, specifically cash collateral as it relates to Legg Mason's hedging activity, that is allowed at any given time per revisions to our revolving credit facility. Line of Credit Facility, Maximum Amount Outstanding During Period Line of Credit Facility, Maximum Amount Outstanding During Period Line of Credit Facility, Interest Rate at Period End Line of Credit Facility, Interest Rate at Period End New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Compensation and benefits [Member] Compensation and benefits [Member] Compensation and benefits [Member] Distribution Service [Member] Distribution Service [Member] Other Expense [Member] Other Expense [Member] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Fair Value, Assets, Level 1 to Level 2 Transfers, Amount Fair Value, Assets, Level 1 to Level 2 Transfers, Amount Fair Value, Assets, Level 2 to Level 1 Transfers, Amount Fair Value, Assets, Level 2 to Level 1 Transfers, Amount Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount Restricted stock and restricted stock unit transactions Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block] Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Abstract] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Other Long-term Investments [Member] Other Long-term Investments [Member] Other Investments [Member] Other Investments [Member] Seed Capital in Proprietary Funds [Member] [Domain] Seed Capital in Proprietary Funds [Member] [Domain] Seed capital investments in proprietary funds. Real Estate Investment [Member] Real Estate Investment [Member] Current Investments [Member] Current Investments [Member] Current Investments [Member] Investments [Member] Investments [Member] Equity method investments in partnerships and Llc [Member] Equity Method Investments [Member] Cost-method Investments [Member] Cost-method Investments [Member] Contingent Consideration Liability Contingent consideration liability [Member] Contingent consideration liability [Member] Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Period Increase (Decrease) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Period Increase (Decrease) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) Derivative Instruments, Gain (Loss) [Table] Derivative Instruments, Gain (Loss) [Table] Designated as Hedging Instrument [Member] Designated as Hedging Instrument [Member] Cash Flow Hedging [Member] Cash Flow Hedging [Member] Forward Contracts [Member] Forward Contracts [Member] Other Nonoperating Income (Expense) [Member] Interest Expense [Member] Interest Expense [Member] Derivative Instruments, Gain (Loss) [Line Items] Derivative Instruments, Gain (Loss) [Line Items] Derivative Instruments Not Designated as Hedging Instruments, Gain Derivative Instruments Not Designated as Hedging Instruments, Gain Derivative Instruments Not Designated as Hedging Instruments, Loss Derivative Instruments Not Designated as Hedging Instruments, Loss Derivative, Gain on Derivative Derivative, Loss on Derivative Management [Member] Management [Member] Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan [Table Text Block] Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan [Table Text Block] Share-based Compensation Arrangements by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding [Table Text Block] Share-based Compensation Arrangements by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding [Table Text Block] Assumptions used to determine the weighted-average fair value of option grants Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Derivative Asset, Fair Value, Gross Asset Derivative Asset, Fair Value, Gross Asset Derivative Asset, Fair Value, Gross Liability Derivative Asset, Fair Value, Gross Liability Derivative Asset Derivative Asset Derivative Asset, Not Subject to Master Netting Arrangement Derivative Asset, Not Subject to Master Netting Arrangement Derivative Asset, Collateral, Obligation to Return Cash, Offset Derivative Asset, Collateral, Obligation to Return Cash, Offset Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement Property Subject to or Available for Operating Lease, by Major Property Class [Table] Property Subject to or Available for Operating Lease, by Major Property Class [Table] Commitments Related to Vacated and Subleased Space [Member] Commitments Related to Vacated and Subleased Space [Member] Portion of the Lease Reserve Liability that is related to space permanently vacated by Legg Mason and subleased by a third party Commitments Related to Vacated Space that Remains Vacant [Member] Commitments Related to Vacated Space that Remains Vacant [Member] Portion of the lease reserve liability balance that has been permanently vacated by Legg Mason and has yet to be subleased by a third party Vacated Office Space [Member] Vacated Office Space [Member] Total lease reserve liability related to space permanently vacated by Legg Mason Property Subject to or Available for Operating Lease [Line Items] Property Subject to or Available for Operating Lease [Line Items] Operating Leases, Future Minimum Payments Due, Future Minimum Sublease Rentals Operating Leases, Future Minimum Payments Due, Future Minimum Sublease Rentals Percentage of Sublease Rentals due from Counterparty Percentage of Sublease Rentals due from Counterparty Percentage of minimum sublease rentals due from a counterparty. SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount Operating Leases, Future Minimum Payments Due SEC Schedule, 12-09, Valuation Allowances and Reserves, Period Increase (Decrease) SEC Schedule, 12-09, Valuation Allowances and Reserves, Period Increase (Decrease) Non-employee directors [Abstract] Non-employee directors [Abstract] Common Stock [Member] Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized ShareBasedCompensationArrangementbyShare-basedPaymentAwardSharesIssued ShareBasedCompensationArrangementbyShare-basedPaymentAwardSharesIssued Number of Shares Issued Under Equity Plan Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Option, Nonvested, Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Option, Nonvested, Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Option, Nonvested, Weighted Average Exercise Price Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Capitalized Contract Cost, Net, Current Capitalized Contract Cost, Net, Current Capitalized Contract Cost, Net, Noncurrent Capitalized Contract Cost, Net, Noncurrent Capitalized Contract Cost, Amortization Capitalized Contract Cost, Amortization Components of intangible assets Schedule of Finite-lived and Indefinite-lived Intangible Assets by Major Class [Table Text Block] Tabular disclosure of the gross carrying amount and accumulated amortization of amortizable intangibles assets and the carrying value of intangible assets not subject to amortization, excluding goodwill, in total and by major class. A major class is composed of intangible assets that can be grouped together because they are similar, either by their nature or by their use in the operations of a company. Estimated amortization expense Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block] Changes in carrying value of goodwill Schedule of Goodwill [Table Text Block] Schedule of Goodwill [Table] Schedule of Goodwill [Table] Schedule of Goodwill by Book Value [Axis] Schedule of Goodwill by Book Value [Axis] This element represents the disclosures required for goodwill, including gross value, accumulated impairment and net book value. Goodwill by Book Value [Domain] Goodwill by Book Value [Domain] This element represents the disclosures required for the purpose of calculating net book value of goodwill. Goodwill Gross Value [Member] Goodwill Gross Value [Member] This element represents the gross book value of the goodwill. Goodwill Accumulated Impairment [Member] Goodwill Accumulated Impairment [Member] This element represents the accumulated loss recognized from the impairment of goodwill. Goodwill Net Value [Member] Goodwill Net Value [Member] This element represents the net book value of the goodwill. Goodwill Goodwill [Line Items] Goodwill, Gross Goodwill, Gross Goodwill, Acquired During Period Goodwill, Acquired During Period Other Intangible Assets, Net Other Intangible Assets, Net Goodwill, Impaired, Accumulated Impairment Loss Goodwill, Impaired, Accumulated Impairment Loss Impact of excess tax basis amortization Goodwill, Subsequent Recognition of Deferred Tax Asset Business Acquisition, Goodwill, Expected Tax Deductible Amount Business Acquisition, Goodwill, Expected Tax Deductible Amount Other, including changes in foreign exchange rates Goodwill, Other Increase (Decrease) Earnings Per Share, Policy [Policy Text Block] Earnings Per Share, Policy [Policy Text Block] RARE Management Team [Member] RARE Management Team [Member] After the acquisition of RARE Infrastructure the RARE Management Team will maintain a noncontrolling interest in the company. TRG [Member] TRG [Member] TRG [Member] Other Affiliates [Member] Other Affiliates [Member] Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Significant Accounting Policies Significant Accounting Policies [Text Block] Equity [Abstract] Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table] Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table] Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Exercise Price Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Exercise Price Net Income (Loss) Attributable to Noncontrolling Interests [Table Text Block] Net Income (Loss) Attributable to Noncontrolling Interests [Table Text Block] Reconciliation of components of Net Income (Loss) Attributable to Noncontrolling Interests [Table Text Block] Redeemable Noncontrolling Interest [Table Text Block] Redeemable Noncontrolling Interest [Table Text Block] Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets, Net Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount Indefinite-lived Intangible Assets (Excluding Goodwill) Award Date [Axis] Award Date [Axis] Award Date [Domain] Award Date [Domain] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense) Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense) Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options Proceeds from Stock Options Exercised Proceeds from Stock Options Exercised Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options AssetClass [Member] AssetClass [Member] Class of asset managed, ie equity, fixed income, alternative, liquidity Disaggregation of Revenue [Table Text Block] Disaggregation of Revenue [Table Text Block] Loss Contingencies [Table] Loss Contingencies [Table] Litigation Case [Axis] Litigation Case [Axis] Litigation Case [Domain] Litigation Case [Domain] Litigation Status [Axis] Litigation Status [Axis] Litigation Status [Domain] Litigation Status [Domain] Loss Contingency Nature [Axis] Loss Contingency Nature [Axis] Loss Contingency, Nature [Domain] Loss Contingency, Nature [Domain] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Litigation Settlement, Amount Awarded to Other Party Litigation Settlement, Amount Awarded to Other Party Payments for Legal Settlements Payments for Legal Settlements Estimated Litigation Liability, Current Estimated Litigation Liability, Current Litigation Settlement, Expense Litigation Settlement, Expense Loss Contingency Accrual Loss Contingency Accrual Loss Contingency Accrual, Provision Loss Contingency Accrual, Provision Exercise Price Range [Axis] Exercise Price Range [Axis] Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Domain] Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Domain] Range of exercise price, 14.81 - 25.00 [Member] [Member] Range of exercise price, 14.81 - 25.00 [Member] [Member] Range of exercise price of LM Stock options of 14.81 - 25.00 [Member] Range of exercise price, 25.01 - 35.00 [Member] Range of exercise price, 25.01 - 35.00 [Member] Represents the range of exercise price in dollars per share from $ 25.01 to 35.00 for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards. Range of exercise price, 35.01 - 55.18 [Member] Range of exercise price, 35.01 - 55.18 [Member] Represents the range of exercise price in dollars per share from $35.01 to $55.18 for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards. Range of exercise price, 35.01 - 94.00 [Member] Range of exercise price, 35.01 - 94.00 [Member] Represents the range of exercise price in dollars per share from $ 35.01 to 94.00 for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards. Stock based compensation disclosure Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period