Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Finite-Lived Intangible Assets by Major Class [Table] Schedule of Finite-Lived Intangible Assets [Table] Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] Construction materials and contracting [Member] Construction materials and contracting [Member] This segment mines aggregates and markets crushed stone, sand, gravel and related construction materials, including ready-mixed concrete, cement, asphalt, liquid asphalt and other value-added products. It also performs integrated contracting services. This segment operates in the central, southern and western United States and Alaska and Hawaii. Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Series of Individually Immaterial Business Acquisitions Series of Individually Immaterial Business Acquisitions [Member] 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] Customer relationships Customer Relationships [Member] Noncompete agreements Noncompete Agreements [Member] Other intangible assets Other Intangible Assets [Member] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Finite-lived Intangible Assets Acquired Finite-lived Intangible Assets Acquired Intangible assets, gross Finite-Lived Intangible Assets, Gross Intangible assets, less accumulated amortization Finite-Lived Intangible Assets, Accumulated Amortization Intangible assets, net (excluding goodwill) Intangible Assets, Net (Excluding Goodwill) Amortization of intangible assets Amortization of Intangible Assets Regulated Operations [Abstract] Public Utilities General Disclosures [Table] Public Utilities General Disclosures [Table] Regulatory Agency [Axis] Regulatory Agency [Axis] Regulatory Agency [Domain] Regulatory Agency [Domain] MTPSC MTPSC [Member] Montana Public Service Commission Public Utility [Axis] Public Utility [Axis] Utility Plant [Domain] Utility Plant [Domain] Electricity [Member] Electricity [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Montana-Dakota Utilities Co. [Member] Montana-Dakota Utilities Co. [Member] Montana-Dakota Utilities Co. [Member], a direct wholly owned subsidiary of MDU Energy Capital Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Public Utilities, General Disclosures [Line Items] Public Utilities, General Disclosures [Line Items] Public Utilities, Approved Rate Increase (Decrease), Amount Public Utilities, Approved Rate Increase (Decrease), Amount Fair Value Disclosures [Abstract] Available-for-sale securities Available-for-sale Securities [Table Text Block] Assets and liabilities measured at fair value on a recurring basis Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Fair value of long term debt outstanding Fair Value, by Balance Sheet Grouping [Table Text Block] Business Combinations [Abstract] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Leases [Abstract] Remainder of 2019 Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year 2020 Lessee, Operating Lease, Liability, Payments, Due Year Two 2021 Lessee, Operating Lease, Liability, Payments, Due Year Three 2022 Lessee, Operating Lease, Liability, Payments, Due Year Four 2023 Lessee, Operating Lease, Liability, Payments, Due Year Five Thereafter Lessee, Operating Lease, Liability, Payments, Due after Year Five Total Lessee, Operating Lease, Liability, Payments, Due Less discount Lessee, Operating Lease, Liability, Undiscounted Excess Amount Total operating lease liabilities Operating Lease, Liability Discontinued Operations and Disposal Groups [Abstract] Discontinued operations Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] WUTC WUTC [Member] WUTC [Member] Washington Utilities and Transportation Commission Gas Distribution [Member] Gas Distribution [Member] Pipeline Cost Recovery Mechanism [Member] Pipeline Cost Recovery Mechanism [Member] Pipeline Cost Recovery Mechanism [Member] Purchased Gas Adjustment [Member] Purchased Gas Adjustment [Member] Purchased Gas Adjustment [Member] Cascade Natural Gas [Member] Cascade Natural Gas [Member] Cascade Natural Gas [Member] Public Utilities, Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Domain] Public Utilities, Regulatory Proceeding [Domain] Pending Rate Case [Member] Pending Rate Case [Member] Pending Rate Case [Member] Public Utilities, Requested Rate Increase (Decrease), Amount Public Utilities, Requested Rate Increase (Decrease), Amount Public Utilities, Requested Rate Increase (Decrease), Percentage Public Utilities, Requested Rate Increase (Decrease), Percentage Public Utilities, Requested Rate Increase (Decrease), Amended, Amount Public Utilities, Requested Rate Increase (Decrease), Amended, Amount Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage Public Utilities, Approved Rate Increase (Decrease), Percentage Public Utilities, Approved Rate Increase (Decrease), Percentage Purchased Gas Costs Recovery Period Regulatory Asset, Amortization Period Investments used to satisfy nonqualified benefit plans obligations Life Insurance, Corporate or Bank Owned, Amount Net unrealized gain (loss) on investments used to satisfy obligations under nonqualified benefit plans Life Insurance, Corporate or Bank Owned, Change in Value Retirement Benefits [Abstract] Pension and other postretirement benefit plans Pension and Other Postretirement Benefits Disclosure [Text Block] Debt Disclosure [Abstract] Long-term debt outstanding Schedule of Long-term Debt Instruments [Table Text Block] Schedule of debt maturities Schedule of Maturities of Long-term Debt [Table Text Block] Segment Reporting [Abstract] Business segment data Segment Reporting, Policy [Policy Text Block] Statement of Financial Position [Abstract] Statement [Table] Statement [Table] Scenario [Axis] Scenario [Axis] Scenario [Domain] Scenario [Domain] Statement [Line Items] Statement [Line Items] Assets Assets [Abstract] Current assets: Assets, Current [Abstract] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Receivables, net Receivables, Net, Current Inventories Inventory, Net Prepayments and other current assets Other Assets, Current Current assets held for sale Disposal Group, Including Discontinued Operation, Assets, Current Total current assets Assets, Current Investments Long-term Investments Property, plant and equipment Property, Plant and Equipment, Gross Less accumulated depreciation, depletion and amortization Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Net property, plant and equipment Property, Plant and Equipment, Net Deferred charges and other assets: Assets, Noncurrent [Abstract] Goodwill Goodwill Other intangible assets, net Operating lease right-of-use assets (Note 11) Operating Lease, Right-of-Use Asset Other Other Assets, Noncurrent Noncurrent assets held for sale Disposal Group, Including Discontinued Operation, Assets, Noncurrent Total deferred charges and other assets Assets, Noncurrent Total assets Assets Liabilities and Stockholders' Equity Liabilities and Equity [Abstract] Current liabilities: Liabilities, Current [Abstract] Short-term borrowings Short-term Debt Long-term debt due within one year Long-term Debt, Current Maturities Accounts payable Accounts Payable, Current Taxes payable Taxes Payable, Current Dividends payable Dividends Payable, Current Accrued compensation Employee-related Liabilities, Current Current operating lease liabilities (Note 11) Operating Lease, Liability, Current Other accrued liabilities Other Liabilities, Current Current liabilities held for sale Disposal Group, Including Discontinued Operation, Liabilities, Current Total current liabilities Liabilities, Current Long-term debt Long-term Debt, Excluding Current Maturities Deferred credits and other liabilities: Liabilities, Noncurrent [Abstract] Deferred income taxes Deferred Income Tax Liabilities, Net Noncurrent operating lease liabilities (Note 11) Operating Lease, Liability, Noncurrent Other Other Liabilities, Noncurrent Total deferred credits and other liabilities Liabilities, Noncurrent Commitments and contingencies Commitments and Contingencies Commitments and contingencies Commitments and Contingencies Disclosure [Abstract] Stockholders' equity: Stockholders' Equity Attributable to Parent [Abstract] Authorized - 500,000,000 shares, $1.00 par value Shares issued - 200,876,334 at September 30, 2019, 196,557,245 at September 30, 2018 and 196,564,907 at December 31, 2018 Common Stock, Value, Issued Other paid-in capital Additional Paid in Capital, Common Stock Retained earnings Retained Earnings (Accumulated Deficit) Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Treasury stock at cost - 538,921 shares Treasury Stock, Common, Value Total stockholders' equity Stockholders' Equity Attributable to Parent Total liabilities and stockholders' equity Liabilities and Equity Earnings Per Share [Abstract] Earnings per common share Earnings Per Share, Policy [Policy Text Block] Business Combination, Consideration Transferred [Abstract] Payments to Acquire Businesses, Gross Payments to Acquire Businesses, Gross Business Combination, Long-Term Debt Assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt Loss Contingencies [Table] Loss Contingencies [Table] Variable Interest Entities [Axis] Variable Interest Entities [Axis] Variable Interest Entity, Classification [Domain] Variable Interest Entity, Classification [Domain] Fuel contract Fuel contract [Member] Fuel contract. Variable Interest Entities [Line Items] Loss Contingencies [Line Items] Variable interest entity, reporting entity involvement, maximum loss exposure, amount Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount Schedule of Goodwill [Table] Schedule of Goodwill [Table] Natural gas distribution Natural gas distribution [Member] This segment distributes natural gas in Montana, North Dakota, South Dakota, Wyoming, Idaho, Minnesota, Oregon and Washington. These operations also supply related value-added services. Construction materials and contracting Construction services Construction services [Member] This segment specializes in constructing and maintaining electric and communication lines, gas pipelines, fire suppression systems, and external lighting and traffic signalization equipment. This segment also provides utility excavation services and inside electrical wiring, cabling and mechanical services, sells and distributes electrical materials, and manufactures and distributes specialty equipment. Goodwill [Line Items] Goodwill [Line Items] Goodwill [Roll Forward] Goodwill [Roll Forward] Balance at beginning of period Goodwill acquired during the year Goodwill, Acquired During Period Measurement period adjustments Goodwill, Purchase Accounting Adjustments Balance at end of period Changes in the carrying amount of goodwill Schedule of Goodwill [Table Text Block] Other amortizable intangible assets Schedule of Finite-Lived Intangible Assets [Table Text Block] Estimated amortization expense Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Business Acquisition [Line Items] Business Acquisition [Line Items] Business Combinations Business Combination Disclosure [Text Block] Receivables [Abstract] Receivables past due 90 days or more Financing Receivable, 90 Days or More Past Due, Still Accruing Allowance for doubtful accounts receivable Accounts Receivable, Allowance for Credit Loss Inventory Disclosure [Abstract] Aggregates held for resale Aggregates Held For Resale Inventory Aggregated amount of aggregates held for resale. This amount is net of valuation reserves and adjustments. Asphalt oil Asphalt oil inventory Aggregated amount of asphalt oil inventory. This amount is net of valuation reserves and adjustments. Materials and supplies Inventory, Raw Materials and Supplies, Gross Merchandise for resale Inventory, Finished Goods, Gross Natural gas in storage (current) Energy Related Inventory, Natural Gas in Storage Other Other Inventory, Net of Reserves Total Natural gas in storage noncurrent Inventory, Gas in Storage Underground, Noncurrent Revenue from Contract with Customer [Abstract] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Electric Electric [Member] This segment generates, transmits and distributes electricity in Montana, North Dakota, South Dakota and Wyoming. These opeartions also supply related value-added services. Pipeline and midstream Pipeline and Midstream [Member] This segment provides natural gas transportation, underground storage, processing and gathering services, as well as oil gathering, through regulated and nonregulated pipeline systems primarily in the Rocky Mountain and northern Great Plains regions of the United States. Other Corporate and Other [Member] Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] Residential utility sales Residential Utility Sales [Member] Residential Utility Sales [Member] Commercial utility sales Commercial Utility Sales [Member] Commercial utility sales [Member] Industrial utility sales Industrial Utility Sales [Member] Industrial Utility Sales [Member] Other utility sales Other Utility Sales [Member] Other utility sales Product and Service [Axis] Product and Service [Axis] Product and Service [Domain] Product and Service [Domain] Natural gas transportation Natural Gas Transportation [Member] Natural Gas Transportation [Member] Natural gas gathering Natural Gas Gathering [Member] Natural Gas Gathering [Member] Natural gas storage Natural Gas, Storage [Member] Contracting services Contracting Services [Member] Contracting Services [Member] Construction materials Construction Materials [Member] Construction Materials [Member] Inside specialty contracting Inside Specialty Contracting [Member] Inside Specialty Contracting [Member] Outside specialty contracting Outside Specialty Contracting [Member] Outside Specialty Contracting [Member] Other Other Revenues [Member] Other revenues from disaggregation by customer or product and service. Revenues from contracts with customers Revenue From Contracts With Customers [Member] Revenue From Contracts With Customers [Member] Revenues out of scope Revenues Out Of Scope [Member] Revenues out of scope of ASC 606, Revenue from Contracts with Customers Subsegments Consolidation Items [Axis] Subsegments Consolidation Items [Axis] Subsegments Consolidation Items [Domain] Subsegments Consolidation Items [Domain] Intrasegment eliminations Intersubsegment Eliminations [Member] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Intersegment eliminations Intersegment Eliminations [Member] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Revenues Revenues Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Regulation Status [Axis] Regulation Status [Axis] Regulated and unregulated operation [Domain] Regulated and Unregulated Operation [Domain] Regulated operation Regulated Operation [Member] Nonregulated operation Unregulated Operation [Member] Total intersegment operating revenues Consolidation, Eliminations [Member] Other Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Operating Income (Loss) Operating Income (Loss) Income from continuing operations Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent Income (loss) from discontinued operations, net of tax Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent Net income Net Income (Loss) Attributable to Parent Discontinued operations Discontinued Operations, Policy [Policy Text Block] Organization, Consolidation and Presentation of Financial Statements [Abstract] Basis of presentation Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Disposal Groups, Including Discontinued Operations [Table] Disposal Groups, Including Discontinued Operations [Table] Ownership [Axis] Ownership [Axis] Ownership [Domain] Ownership [Domain] WBI Energy WBI Energy [Member] WBI Energy. Dakota Prairie Refining, LLC Dakota Prairie Refining, LLC [Member] Dakota Prairie Refining, LLC. Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Previous percentage of ownership Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Senior Notes Senior Notes [Member] Commercial Paper Commercial Paper [Member] Term Loan Agreements Loans Payable [Member] Credit Agreements Line of Credit [Member] Medium-term Notes Medium-term Notes [Member] Other Notes Notes Payable to Banks [Member] Long-term Debt Long-term Debt [Member] Long-term debt outstanding [Line Items] Debt Instrument [Line Items] Weighted Average Interest Rate Long-term Debt, Weighted Average Interest Rate, at Point in Time Long-term debt Long-term Debt Unamortized Debt Issuance Costs Debt Issuance Costs, Net Discount Debt Instrument, Unamortized Discount Regulatory Assets and Liabilities Disclosure [Abstract] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Regulatory Asset [Axis] Regulatory Asset [Axis] Regulatory Asset [Domain] Regulatory Asset [Domain] Pension and postretirement benefits Pension and Other Postretirement Plans Costs [Member] Natural gas costs recoverable through rate adjustments Asset Recoverable Gas Costs [Member] Asset Retirement Obligation Costs Asset Retirement Obligation Costs [Member] Cost recovery mechanisms Regulatory Clause Revenues, under-recovered [Member] Manufactured gas plant site remediation Environmental Restoration Costs [Member] Taxes recoverable from/refundable to customers Deferred Income Tax Charge [Member] Plant removal Removal Costs [Member] Conservation programs Renewable Energy Program [Member] Long-term debt refinancing costs Loss on Reacquired Debt [Member] Costs related to identifying generation development Costs related to identifying generation development [Member] Other Regulatory Assets/Liabilities Other Regulatory Assets (Liabilities) [Member] Regulatory Liability [Axis] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Natural gas costs refundable through rate adjustments Liabilities refundable gas costs [Member] Carrying amount as of the balance sheet date of gas costs of a regulated entity that are refundable through future rate adjustments within one year of the balance sheet date or the normal operating cycle, if longer Regulatory Assets and Liabilities Schedule of Regulatory Assets and Liabilities [Line Items] Estimated Recovery Period Regulatory Asset Recovery Assessments Regulatory Assets Regulatory Assets Regulatory Liabilities Regulatory Liabilities Net Regulatory Position Net Regulatory Assets Regulatory assets not earning a rate of return Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided Earnings per common share Earnings Per Share [Text Block] Contingencies Commitments and Contingencies, Policy [Policy Text Block] Variable interest entity Consolidation, Variable Interest Entity, Policy [Policy Text Block] Investments, Fair Value Disclosure Investments, Fair Value Disclosure Asset Impairment Charges Asset Impairment Charges Revenue from contracts with customers Revenue [Policy Text Block] Operating lease cost Operating Lease, Cost Variable lease cost Variable Lease, Cost Short-term lease cost Short-term Lease, Cost Total lease costs Lease, Cost Weighted average remaining lease term Operating Lease, Weighted Average Remaining Lease Term Weighted average discount rate Operating Lease, Weighted Average Discount Rate, Percent Cash paid for amounts included in the measurement of lease liabilities Operating Lease, Payments Fair value measurements Fair Value Disclosures [Text Block] Schedule of Short-term Debt [Table] Schedule of Short-term Debt [Table] Centennial Energy Holdings, Inc. [Member] Centennial Energy Holdings, Inc. [Member] Centennial Energy Holdings, Inc. [Member] Short-term Debt, Type [Axis] Short-term Debt, Type [Axis] Short-term Debt, Type [Domain] Short-term Debt, Type [Domain] Short-term Debt [Line Items] Short-term Debt [Line Items] Accounts receivable and allowance for doubtful accounts Financing Receivables [Text Block] FERC [Member] FERC [Member] Federal Energy Regulatory Commission MISO [Member] MISO [Member] MISO [Member], Midcontinent Independent System Operator, Inc. WBI Energy Transmission, Inc. [Member] WBI Energy Transmission, Inc. [Member] WBI Energy Transmission, Inc. [Member], an indirect wholly owned subsidiary of WBI Holdings, Inc. Income Tax Authority [Axis] Income Tax Authority [Axis] Income Tax Authority [Domain] Income Tax Authority [Domain] Tax Cuts and Jobs Act [Member] Tax Cuts and Jobs Act [Member] Tax Cuts and Jobs Act - TCJA. Transmission Formula Revenue Requirement Transmission Formula Revenue Requirement Transmission Formula Revenue Requirement Supplemental Cash Flow Information [Abstract] Cash flow information Cash Flow, Supplemental Disclosures [Text Block] New Accounting Pronouncements or Change in Accounting Principle [Abstract] New Accounting Pronouncements or Change in Accounting Principle [Abstract] New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Accounting Standards Update 2016-02 [Member] Accounting Standards Update 2016-02 [Member] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [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 Disposal Group Classification [Axis] Disposal Group Classification [Axis] Disposal Group Classification [Domain] Disposal Group Classification [Domain] Discontinued operations, held-for-sale or disposed of by sale Discontinued Operations, Held-for-sale or Disposed of by Sale [Member] Assets [Abstract] Receivables, net Disposal Group, Including Discontinued Operation, Accounts, Notes and Loans Receivable, Net Income taxes receivable (a) Disposal Group, Including Discontinued Operation, Income Taxes Receivable, Assets, Current Amount classified as income taxes receivable attributable to disposal group, expected to be disposed of within one year or the normal operating cycle, if longer. Total current assets held for sale Noncurrent assets: Disposal Group, Including Discontinued Operation, Assets, Noncurrent [Abstract] Deferred income taxes Disposal Group, Including Discontinued Operation, Deferred Tax Assets Other Disposal Group, Including Discontinued Operation, Other Assets, Noncurrent Total noncurrent assets held for sale Total assets held for sale Disposal Group, Including Discontinued Operation, Assets Liabilities [Abstract] Liabilities [Abstract] Accounts payable Disposal Group, Including Discontinued Operation, Accounts Payable, Current Taxes payable Disposal Group, Including Discontinued Operation, Taxes Payable, Current Amount classified as taxes payable attributable to disposal group held for sale or disposed of, expected to be disposed of within one year or the normal operating cycle, if longer. Other accrued liabilities Disposal Group, Including Discontinued Operation, Other Liabilities, Current Total current liabilities held for sale Noncurrent liabilities: Deferred income taxes (b) Disposal Group, Including Discontinued Operation, Deferred Tax Liabilities Total noncurrent liabilities held for sale Disposal Group, Including Discontinued Operation, Liabilities, Noncurrent Total liabilities held for sale Disposal Group, Including Discontinued Operation, Liabilities Goodwill and other intangible assets Goodwill and Intangible Assets Disclosure [Text Block] Fair value measurements Fair Value of Financial Instruments, Policy [Policy Text Block] Investments Investment, Policy [Policy Text Block] Lessee, Leases [Policy Text Block] Lessee, Leases [Policy Text Block] Schedule of Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Guarantor obligations, maximum exposure, undiscounted Guarantor Obligations, Maximum Exposure, Undiscounted Fixed maximum amounts guaranteed by year 2019 Fixed maximum amounts guaranteed by year one Fixed maximum amounts guaranteed by year one. Fixed maximum amounts guaranteed by year 2020 Fixed maximum amounts guaranteed by year two Fixed maximum amounts guaranteed by year two. Fixed maximum amounts guaranteed by year 2021 Fixed maximum amounts guaranteed by year three Fixed maximum amounts guaranteed by year three. Fixed maximum amounts guaranteed by year 2022 Fixed maximum amounts guaranteed by year four Fixed maximum amounts guaranteed by year four. Fixed maximum amounts guaranteed by year 2023 Fixed Maximum Amounts Guaranteed By Year Five Fixed maximum amounts guaranteed by year five. Fixed maximum amounts guaranteed, thereafter Fixed Maximum Amounts Guaranteed, Thereafter Fixed maximum amounts guaranteed, thereafter. No scheduled maturity date Fixed maximum amounts guaranteed by year no maturity date Fixed maximum amounts guaranteed with no maturity date. Amount outstanding under guarantees that is reflected on balance sheet Amount outstanding under guarantees that is reflected on balance sheet The amount outstanding under gurantees that is reflected on the balance sheet. Letters of credit Letters of credit Letters of credit. Letters of credit set to expire - 2019 Letters of Credit Set to Expire Current Year Letters of credit set to expire current year. Letters of credit set to expire - 2020 Letters of Credit Set to Expire in Next Fiscal Year Letters of credit set to expire in next fiscal year. Outstanding letters of credit Letters of Credit Outstanding, Amount Amount of surety bonds outstanding Amount of surety bonds outstanding Amount of surety bonds outstanding related to construction contracts and reclamation obligations. Accounts receivable and allowance for doubtful accounts Receivables, Trade and Other Accounts Receivable, Allowance for Doubtful Accounts, Policy [Policy Text Block] OPUC OPUC [Member] Oregon Public Utility Commission Approved Customer Refunds Public Utilities, Approved One-Time Increase (Decrease) to Customers, Amount Public Utilities, Approved One-Time Increase (Decrease) to Customers, Amount [Line Items] Contingencies Commitments and Contingencies Disclosure [Text Block] Weighted average common shares outstanding Schedule of Weighted Average Number of Shares [Table Text Block] Equity [Abstract] Comprehensive income (loss) Comprehensive Income (Loss) Note [Text Block] Reclassification out of Accumulated Other Comprehensive Income [Table] Reclassification out of Accumulated Other Comprehensive Income [Table] Equity Components [Axis] Equity Components [Axis] Total accumulated other comprehensive loss Equity Component [Domain] Reclassification adjustment for loss on derivative instruments included in net income Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] Amortization of postretirement liability losses included in net periodic benefit cost Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Reclassification adjustment for foreign currency translation adjustment included in net income Accumulated Foreign Currency Adjustment Attributable to Parent [Member] Reclassification adjustment on available-for-sale investments included in net income AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification out of accumulated other comprehensive income Reclassification out of Accumulated Other Comprehensive Income [Member] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Interest rate contract Interest Rate Contract [Member] Reclassification adjustment out of accumulated other comprehensive income [Line Items] Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Interest expense Interest Expense Income taxes Income Tax Expense (Benefit) Other income Other Nonoperating Income (Expense) Net Income Weighted average common shares outstanding - basic Weighted Average Number of Shares Outstanding, Basic Effect of dilutive performance share awards and restricted stock units Weighted Average Number Diluted Shares Outstanding Adjustment Weighted average common shares outstanding - diluted Weighted Average Number of Shares Outstanding, Diluted Shares excluded from the calculation of diluted earnings per share Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Dividends declared per common share Common Stock, Dividends, Per Share, Declared Operating leases future minimum lease payments due [Abstract] Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract] 2019 Operating Leases, Future Minimum Payments Due, Next Twelve Months 2020 Operating Leases, Future Minimum Payments, Due in Two Years 2021 Operating Leases, Future Minimum Payments, Due in Three Years 2022 Operating Leases, Future Minimum Payments, Due in Four Years 2023 Operating Leases, Future Minimum Payments, Due in Five Years Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Regulatory Assets and Liabilities Schedule of Regulatory Assets and Liabilities [Text Block] Regulatory Assets Schedule of Regulatory Assets [Table Text Block] Regulatory Liabilities Schedule of Regulatory Liabilities [Table Text Block] Statement of Comprehensive Income [Abstract] Reclassification adjustment for loss on derivative instruments included in net income, tax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax Amortization of postretirement liability losses included in net periodic benefit cost, tax Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax Foreign currency translation adjustments recognized during the period, tax Other Comprehensive Income (Loss), Foreign Currency Translation Gain (Loss) Arising During Period, Tax Reclassification of foreign currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Tax Net unrealized gain (loss) on available-for-sale investments arising during the period, tax Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax Reclassification adjustment for (gain) loss on available-for-sale investments included in net income, tax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax Common stock, shares issued Common Stock, Shares, Issued Treasury shares Treasury Stock, Common, Shares Common stock, shares authorized Common Stock, Shares Authorized Common stock, par value Common Stock, Par or Stated Value Per Share Income Statement [Abstract] Operating revenues: Revenues [Abstract] Operating revenues: Operating expenses: Operating Expenses [Abstract] Operation and maintenance: Selling, General and Administrative Expense Purchased natural gas sold Utilities Operating Expense, Gas and Petroleum Purchased Depreciation, depletion and amortization Depreciation, Depletion and Amortization Taxes, other than income Taxes, Miscellaneous Electric fuel and purchased power Utilities Operating Expense, Purchased Power Total operating expenses Operating Expenses Operating income Interest expense Income before income taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Income taxes Income from continuing operations Income (loss) from discontinued operations, net of tax (Note 10) Net income Earnings per share - basic: Earnings Per Share, Basic [Abstract] Income from continuing operations Income (Loss) from Continuing Operations, Per Basic Share Discontinued operations, net of tax Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Basic Share Earnings per share - basic Earnings Per Share, Basic Earnings per share - diluted: Earnings Per Share, Diluted [Abstract] Income from continuing operations Income (Loss) from Continuing Operations, Per Diluted Share Discontinued operations, net of tax Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Diluted Share Earnings per share - diluted Earnings Per Share, Diluted Weighted average common shares outstanding - diluted Information on the Company's businesses Schedule of Segment Reporting Information, by Segment [Table Text Block] Fair value, assets and liabilities measured on recurring and nonrecurring basis [Table] Fair Value, Recurring and Nonrecurring [Table] Fair value by measurement frequency [Axis] Measurement Frequency [Axis] Fair value by measurement frequency [Domain] Measurement Frequency [Domain] Fair value, measurements, recurring Fair Value, Recurring [Member] Fair value, hierarchy [Axis] Fair Value Hierarchy and NAV [Axis] Fair value, measurements [Domain] Fair Value Hierarchy and NAV [Domain] Fair value, inputs, level 2 Fair Value, Inputs, Level 2 [Member] Assets [Axis] Asset Class [Axis] Total assets measured at fair value Asset Class [Domain] Money market funds Money Market Funds [Member] Insurance contract Cash Surrender Value [Member] Mortgage-backed securities Collateralized Mortgage Backed Securities [Member] U.S. Treasury securities US Treasury Securities [Member] Fair value measurements [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Assets, fair value disclosure Assets, Fair Value Disclosure Concentration risks, percentage [Abstract] Concentration Risks, Percentage [Abstract] Concentration Risks, Percentage Percentage in fixed-income and other investments Percentage in fixed-income and other investments The percentage of insurance contracts in fixed-income and other investments. Percentage investment in common stock of large-cap companies Percentage investment in common stock of large-cap companies The percentage of insurance contracts in common stock of large-cap companies. Percentage investment in common stock of mid-cap companies Percentage investment in common stock of mid-cap companies The percentage of insurance contracts in common stock of mid-cap companies. Percentage investment in common stock of small-cap companies Percentage investment in common stock of small-cap companies The percentage of insurance contracts in common stock of small-cap companies. Percentage investment in target date investments Percentage Investment in Target Date Investments The percentage of insurance contracts in target date investments. Percentage investment in cash and cash equivalents Percentage Investment in Cash and Cash Equivalents Percentage of insurance contracts in cash and cash equivalents. Lease, Cost [Table Text Block] Lease, Cost [Table Text Block] Lessee, Operating Lease, Liability, Maturity [Table Text Block] Lessee, Operating Lease, Liability, Maturity [Table Text Block] Site Contingency [Table] Site Contingency [Table] Statistical Measurement [Axis] Statistical Measurement [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Environmental Remediation Site [Axis] Environmental Remediation Site [Axis] Environmental Remediation Site [Domain] Environmental Remediation Site [Domain] Portland Harbor Site Portland Harbor Site [Member] Information related to the Portland Harbor site clean-up. Missoula, MT manufactured gas plant site Missoula, MT manufactured gas plant site [Member] Missoula, MT manufactured gas plant site [Member] Bremerton, WA manufactured gas plant site Bremerton, WA Manufactured Gas Plant Site [Member] Claim #2 - Contamination at a manufactured gas plant site in Washington received in 1997. Bellingham, WA manufactured gas plant site Bellingham, WA Manufactured Gas Plant Site [Member] Claim #3 - Contamination at a manufactured gas plant site in Washington owned from 1946-1962 and received in 2008. Site Contingency [Line Items] Site Contingency [Line Items] Environmental matters investigative costs Environmental matters investigative costs Estimated investigative costs. Environmental matters, estimated costs Environmental Matters, Estimated Costs Environmental matters, estimated costs. Total estimated costs for site remediation Total Estimated Costs For Site Remediation Total estimated costs for site remediation Environmental matters accrual for site remediation Environmental Matters Accrual For Site Remediation Environmental matters accrual for site remediation Site contingency, loss exposure not accrued, best estimate Site Contingency, Loss Exposure Not Accrued, Best Estimate Total estimated costs for site remedial investigation and feasibility study Total Estimated Costs For Site Remedial Investigation And Feasibility Study Total estimated costs for site remedial investigation and feasibility study Incurred costs for site remedial investigation and feasibility study Incurred Costs For Site Remedial Investigation And Feasibility Study Incurred costs for site remedial investigation and feasibility study Environmental matters accrual of investigative costs Environmental Matters Accrual For Remedial Investigation and Feasibility Study Costs Environmental matters accrual for remedial investigation and feasibility study costs. Site contingency, loss exposure not accrued, preferred alternative estimate Site Contingency, Loss Exposure Not Accrued, Preferred Alternative Estimate Preferred alternative cost estimate of the loss exposure for reasonably possible environmental contingencies at an individual site Business segment data Segment Reporting Disclosure [Text Block] WYPSC WYPSC [Member] Wyoming Public Service Commission. New Accounting Pronouncements and Changes in Accounting Principles [Abstract] New accounting standards New Accounting Pronouncements and Changes in Accounting Principles [Text Block] Accumulated comprehensive loss Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Reclassification out of accumulated other comprehensive loss Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Entities [Table] Entity Information [Line Items] Entity Information [Line Items] Document type Document Type Document Quarterly Report Document Quarterly Report Document Period End Date Document Period End Date Document Transition Report Document Transition Report Entity File Number Entity File Number Entity registrant name Entity Registrant Name Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Entity Tax Identification Number Entity Tax Identification Number Entity Address, Address Line One Entity Address, Address Line One Entity Address, Address Line Two Entity Address, Address Line Two Entity Address, City or Town Entity Address, City or Town Entity Address, State or Province Entity Address, State or Province Entity Address, Postal Zip Code Entity Address, Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Title of 12(b) Security Title of 12(b) Security Trading Symbol Trading Symbol Security Exchange Name Security Exchange Name Entity Current Reporting Status Entity Current Reporting Status Entity Interactive Data Current Entity Interactive Data Current Entity Filer Category Entity Filer Category Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Shell Company Entity Shell Company Entity common stock, shares outstanding Entity Common Stock, Shares Outstanding Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation, Amount Revenue, Remaining Performance Obligation, Amount Regulatory matters Public Utilities Disclosure [Text Block] Business Combinations Policy [Policy Text Block] Business Combinations Policy [Policy Text Block] New accounting standards New Accounting Pronouncements, Policy [Policy Text Block] Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Adjustments for New Accounting Principle, Early Adoption [Member] Adjustments for New Accounting Principle, Early Adoption [Member] Equity component [Domain] Net unrealized gain (loss) on derivative instruments qualifying as hedges Postretirement liability adjustment Foreign currency translation adjustment Net unrealized gain (loss) on available-for-sale investments Total accumulated other comprehensive loss AOCI Attributable to Parent [Member] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated other comprehensive income (loss) [Roll Forward] AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] Balance Other comprehensive income before reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Amounts reclassified (to) from accumulated other comprehensive loss Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Other comprehensive income Other Comprehensive Income (Loss), Net of Tax Reclassification adjustment of prior period tax effects related to TCJA included in accumulated other comprehensive loss Reclassification from AOCI, Current Period, Tax Balance NDPSC NDPSC [Member] North Dakota Public Service Commission. Fair value, by balance sheet grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Measurement Basis [Axis] Measurement Basis [Axis] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Portion at fair value measurement Portion at Fair Value Measurement [Member] Carrying Amount Reported Value Measurement [Member] Fair Value Estimate of Fair Value Measurement [Member] Fair value, balance sheet grouping [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Long-term debt Long-term debt, fair value Long-term Debt, Fair Value Cash expenditures for interest and income taxes and noncash investing and financing transactions Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] 2018 Acquisitions [Member] 2018 Acquisitions [Member] 2018 Acquisitions [Member] Receivables, net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables Inventories Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory Prepayments and other current assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other Total current assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets Property, plant and equipment Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment Other Intangible assets, net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles Other Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill Total deferred charges and other assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets Total assets acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets Current liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities Asset retirement obligation Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other Deferred income taxes Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities Total deferred credits and other liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities Total liabilities assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities Total consideration (fair value) Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net Measurement Period Adjustments [Abstract] Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract] Receivables, net Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Inventory Inventories Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Assets Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Assets Prepayments and other current assets Business Combination, Provisional Information, Initial Accounting Incomplete, Total Current Assets Business Combination, Provisional Information, Initial Accounting Incomplete, Total Current Assets Property, plant and equipment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Property, Plant, and Equipment Goodwill Total deferred charges and other assets Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Total Deferred Assets Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Total Deferred Assets Total assets acquired Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Assets Current liabilities Business Combination, Provisional Information, Initial Accounting Incomplete, Current Liabilities Business Combination, Provisional Information, Initial Accounting Incomplete, Current Liabilities Total liabilities assumed Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Liabilities Inventories and natural gas in storage Inventory, Policy [Policy Text Block] Disposal groups, including discontinued operations Disposal Groups, Including Discontinued Operations [Table Text Block] Reconciliation of major classes of income and expense Reconciliation of Major Classes of Income And Expense [Table Text Block] Reconciliation of major classes of income and expense related to the disposal group. Statement of Stockholders' Equity [Abstract] Common stock Common Stock [Member] Other paid-in capital Additional Paid-in Capital [Member] Retained earnings Retained Earnings [Member] Accumulated other comprehensive loss Treasury stock Treasury Stock [Member] Accounting Standards Update 2014-09 Accounting Standards Update 2014-09 [Member] Adjustments for new accounting pronouncement Balance (in shares) Treasury stock (in shares) Treasury Stock, Shares Cumulative effect of adoption of ASU 2014-09 Cumulative Effect of New Accounting Principle in Period of Adoption Reclassification of certain prior period tax effects from accumulated other comprehensive loss Cumulative Effect on Retained Earnings, Tax Dividends declared on common stock Dividends, Common Stock, Cash Stock-based compensation APIC, Share-based Payment Arrangement, Recognition and Exercise Repurchase of common stock Treasury Stock, Shares, Acquired Repurchase of common stock Treasury Stock, Value, Acquired, Cost Method Share Based Compensation, Shares, Shares Issued Net Of Tax Withholdings Share Based Compensation, Shares, Shares Issued Net Of Tax Withholdings Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings, shares. Share Based Compensation Value Issued Shares Net Of Tax Withholdings Share Based Compensation Value Issued Shares Net Of Tax Withholdings The value of share based compensation shares issued as compensation, net of the value of shares for the payment of tax withholdings. Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings Shares Issued, Value, Share-based Payment Arrangement, before Forfeiture Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings (in shares) Treasury Stock, Shares, Retired Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings Treasury Stock, Retired, Cost Method, Amount Issuance of common stock upon vesting of stock-based compensation, net of shares used for tax withholdings Payment, Tax Withholding, Share-based Payment Arrangement Issuance of common stock (shares) Stock Issued During Period, Shares, New Issues Issuance of common stock Stock Issued During Period, Value, New Issues Balance (in shares) Treasury stock (in shares) Contract assets Contract with Customer, Asset, after Allowance for Credit Loss Change in contract assets Contract with Customer, Asset, Net Current Change Amount of increase (decrease) in current contract assets, net Contract liabilities - current Contract with Customer, Liability, Current Change in contract liabilities - current Contract with Customer, Liability, Current Change Amount of increase (decrease) in current contract liabilities Contract liabilities - noncurrent Contract with Customer, Liability, Noncurrent Change in contract liabilities - noncurrent Contract with Customer Liability, Noncurrent Change Amount of increase (decrease) in noncurrent contract liabilities Net contract assets Contract with Customer, Assets and Liabilities, Net Total of net contract assets and liabilities Change in net contract assets Contract with Customer, Assets and Liabilities, Net Change Amount of increase (decrease) in net contract assets and liabilities Amounts included in contract liability at the beginning of the period Contract with Customer, Liability, Revenue Recognized Amounts from performance obligations satisfied in prior periods Contract with Customer, Performance Obligation Satisfied in Previous Period Debt Instrument Redemption [Table] Debt Instrument Redemption [Table] Debt Instrument, Redemption, Period [Axis] Debt Instrument, Redemption, Period [Axis] Debt Instrument, Redemption, Period [Domain] Debt Instrument, Redemption, Period [Domain] Long-term debt maturities [Line Items] Debt Instrument, Redemption [Line Items] Remainder of 2019 Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year 2020 Long-term Debt, Maturities, Repayments of Principal in Year Two 2021 Long-term Debt, Maturities, Repayments of Principal in Year Three 2022 Long-term Debt, Maturities, Repayments of Principal in Year Four 2023 Long-term Debt, Maturities, Repayments of Principal in Year Five Thereafter Long-term Debt, Maturities, Repayments of Principal after Year Five Basis of presentation Basis of Accounting, Policy [Policy Text Block] Schedule of defined benefit plans disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Retirement Plan Tax Status [Axis] Retirement Plan Tax Status [Axis] Retirement Plan Tax Status [Domain] Retirement Plan Tax Status [Domain] Qualified plan Qualified Plan [Member] Nonqualified plan Nonqualified Plan [Member] Retirement Plan Funding Status [Axis] Retirement Plan Funding Status [Axis] Retirement Plan Funding Status [Domain] Retirement Plan Funding Status [Domain] Underfunded plan Underfunded Plan [Member] Overfunded Plan [Member] Overfunded Plan [Member] Unfunded plan Unfunded Plan [Member] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Pension benefits Pension Plan [Member] Other postretirement benefits Other Postretirement Benefits Plan [Member] Supplemental employee retirement plans Supplemental Employee Retirement Plan [Member] Defined benefit plan disclosure, net periodic benefit cost [Line Items] Defined Benefit Plan Disclosure [Line Items] Service cost Defined Benefit Plan, Service Cost Interest cost Defined Benefit Plan, Interest Cost Expected return on assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Amortization of prior service credit Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Amortization of net actuarial loss Defined Benefit Plan, Amortization of Gain (Loss) Net periodic benefit cost (credit), including amount capitalized Defined Benefit Plan Net Periodic Benefit Cost Including Amount Capitalized The total amount of net periodic benefit cost including amount capitalized for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments), including amount capitalized. Less amount capitalized Defined Benefit Plan Net Periodic Benefit Cost Amount Capitalized The total amount of net periodic benefit cost for defined benefit plans capitalized for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments). Net periodic benefit cost (credit) Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Disaggregation of revenue Disaggregation of Revenue [Table Text Block] Contract balances Contract with Customer, Asset and Liability [Table Text Block] MNPUC [Member] MNPUC [Member] Minnesota Public Utilities Commission Great Plains Natural Gas Co. [Member] Great Plains Natural Gas Co. [Member] Great Plains Natural Gas Co. [Member], a public utility division of Montana-Dakota Public Utilities, Interim Rate Increase (Decrease), Amount Public Utilities, Interim Rate Increase (Decrease), Amount Public Utilities, Interim Rate Increase (Decrease), Percentage Public Utilities, Interim Rate Increase (Decrease), Percentage Operating revenues Disposal Group, Including Discontinued Operation, Revenue Operating expenses Disposal Group, Including Discontinued Operation, Operating Expense Operating income (loss) Disposal Group, Including Discontinued Operation, Operating Income (Loss) Other income Disposal Group, Including Discontinued Operation, Other Income Interest expense Disposal Group, Including Discontinued Operation, Interest Expense Income (loss) from discontinued operations before income taxes Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax Income taxes Discontinued Operation, Tax Effect of Discontinued Operation Income (loss) from discontinued operations Inventories Schedule of Inventory, Current [Table Text Block] Inventories and natural gas in storage Inventory Disclosure [Text Block] Revenue from contracts with customers Revenue from Contract with Customer [Text Block] Other Comprehensive Income Location [Axis] Other Comprehensive Income Location [Axis] Other Comprehensive Income Location [Domain] Other Comprehensive Income Location [Domain] Other comprehensive income: Other Comprehensive Income (Loss), Net of Tax [Abstract] Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $36 and $55 for the three months ended and $(177) and $164 for the nine months ended in 2019 and 2018, respectively Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax Postretirement liability adjustment: Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax [Abstract] Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $95 and $142 for the three months ended and $284 and $442 for the nine months ended in 2019 and 2018, respectively Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, after Tax Foreign currency translation adjustment: Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, before Tax, Portion Attributable to Parent [Abstract] Foreign currency translation adjustment recognized during the period, net of tax of $0 and $0 for the three months ended and $0 and $(14) for the nine months ended in 2019 and 2018, respectively Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss) Arising During Period, Net of Tax Reclassification adjustment for foreign currency translation adjustment included in net income, net of tax of $0 and $0 for the three months ended and $0 and $75 for the nine months ended in 2019 and 2018, respectively Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Net of Tax Foreign currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Net unrealized gain (loss) on available-for-sale investments: Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract] Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax of $3 and $(13) for the three months ended and $35 and $(52) for the nine months ended in 2019 and 2018, respectively Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax Reclassification adjustment for (gain) loss on available-for-sale investments included in net income, net of tax of $(1) and $9 for the three months ended and $10 and $26 for the nine months ended in 2019 and 2018, respectively Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax Net unrealized gain (loss) on available-for-sale investments Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax Other comprehensive income Comprehensive income attributable to common stockholders Comprehensive Income (Loss), Net of Tax, Attributable to Parent Long-term debt issued [Abstract] Long-term debt issued [Abstract] Intermountain Gas Company [Member] Intermountain Gas Company [Member] Intermountain Gas Company [Member] Long-term debt issued [Line Items] Statement of Cash Flows [Abstract] Operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Income from continuing operations Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Deferred income taxes Deferred Income Tax Expense (Benefit) Changes in current assets and liabilities, net of acquisitions: Increase (Decrease) in Operating Capital [Abstract] Receivables Increase (Decrease) in Receivables Inventories Increase (Decrease) in Inventories Other current assets Increase (Decrease) in Other Current Assets Accounts payable Increase (Decrease) in Accounts Payable Other current liabilities Increase (Decrease) in Other Current Liabilities Other noncurrent changes Increase (Decrease) in Other Noncurrent Assets and Liabilities, Net Net cash provided by continuing operations Net Cash Provided by (Used in) Operating Activities, Continuing Operations Net cash used in discontinued operations Cash Provided by (Used in) Operating Activities, Discontinued Operations Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Capital expenditures Payments to Acquire Productive Assets Acquisitions, net of cash acquired Payments to Acquire Businesses, Net of Cash Acquired Net proceeds from sale or disposition of property and other Proceeds from Sale of Productive Assets Investments Payments for (Proceeds from) Investments Net cash used in continuing operations Net Cash Provided by (Used in) Investing Activities, Continuing Operations Net cash provided by discontinued operations Cash Provided by (Used in) Investing Activities, Discontinued Operations Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities Financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Issuance of short-term borrowings Proceeds from Short-term Debt Repayment of short-term borrowings Repayments of Short-term Debt Issuance of long-term debt Proceeds from Issuance of Long-term Debt Repayment of long-term debt Repayments of Long-term Debt Proceeds from issuance of common stock Proceeds from Issuance of Common Stock Dividends paid Payments of Dividends Repurchase of common stock Payments for Repurchase of Common Stock Tax withholding on stock-based compensation Net cash provided by financing activities Net Cash Provided by (Used in) Financing Activities Effect of exchange rate changes on cash and cash equivalents Effect of Exchange Rate on Cash and Cash Equivalents Increase in cash and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and cash equivalents -- beginning of year Cash and cash equivalents -- end of period Seasonality of operations Nature of Operations [Text Block] Litigation Case [Axis] Litigation Case [Axis] Litigation [Domain] Litigation Case [Domain] Loss Contingencies [Line Items] Potential liabilities related to litigation and environmental matters Loss Contingency Accrual Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] 2019 Finite-Lived Intangible Assets, Amortization Expense, Remainder of Fiscal Year 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Three 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Four 2023 Finite-Lived Intangible Assets, Amortization Expense, Year Five Thereafter Finite-Lived Intangible Assets, Amortization Expense, after Year Five Debt Debt Disclosure [Text Block] Schedule of net benefit costs Schedule of Net Benefit Costs [Table Text Block] Leases of Lessee Disclosure [Text Block] Leases of Lessee Disclosure [Text Block] Leases of Lessor Disclosure [Text Block] Leases of Lessor Disclosure [Text Block] Schedule of available-for-sale securities [Table] Schedule of Available-for-sale Securities [Table] Schedule of available-for-sale securities, major types of debt and equity securities [Axis] Financial Instrument [Axis] Major types of debt and equity securities [Domain] Financial Instruments [Domain] Available-for-sale securities [Line Items] Debt Securities, Available-for-sale [Line Items] Available-for-sale securities [Abstract] Debt Securities, Available-for-sale [Abstract] Cost Available-for-sale Securities, Amortized Cost Basis Gross Unrealized Gains Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Losses Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Fair Value Available-for-sale Securities Operating Activities [Axis] Operating Activities [Axis] Operating Activities [Domain] Operating Activities [Domain] Continuing and discontinued operations Continuing and discontinued operations [Member] Continuing and discontinued operations. Interest, net Interest Paid, Excluding Capitalized Interest, Operating Activities Income taxes paid, net Income Taxes Paid, Net AFUDC borrowed Capitalized Interest Costs, Including Allowance for Funds Used During Construction Right-of-use assets obtained in exchange for new operating lease liabilities Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Property, plant and equipment additions in accounts payable Capital Expenditures Incurred but Not yet Paid Debt assumed in connection with a business combination Noncash or Part Noncash Acquisition, Debt Assumed Issuance of common stock in connection with a business combination Stock Issued Operating Lease, Lease Income Operating Lease, Lease Income Lessor, Operating Lease, Payments to be Received, Next Twelve Months Lessor, Operating Lease, Payments to be Received, Next Twelve Months