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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-Section 50
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Percentage of public utility's amended requested rate increase (decrease) with regulatory agency.

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Percentage of public utility's requested return on equity.

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-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
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Regulation and Rates (Details) $ in Thousands					3 Months Ended		4 Months Ended	6 Months Ended		12 Months Ended
Regulation and Rates (Details) $ in Thousands	Dec. 29, 2017 USD ($)	Dec. 19, 2017 USD ($)	Jan. 13, 2017	Jan. 01, 2017	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Apr. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)
Regulatory Assets [Line Items]
Income tax (benefit) expense					$ 3,469	$ 13,122	$ 34,600	$ 15,272	$ 75,665
Subsidiaries [Member]
Regulatory Assets [Line Items]
Public Utilities, Rate Case, Deferred Storm Costs Threshold											$ 10,000
Public Utilities, Rate Case, Deferred Storm Qualifying Costs											500
Income tax (benefit) expense					7,004	22,794		28,696	$ 94,591
Subsidiaries [Member] \| Electric [Member]
Regulatory Assets [Line Items]
Storm Damage Costs Incurred During Period					8,900	20,800
Storm Damage Costs Deferred During Period					0	$ 12,100
Public Utilities, Rate Case, Deferred Storm Costs Threshold											$ 8,000
Decoupling Mechanism [Member]
Regulatory Assets [Line Items]
Deferred Revenue, Revenue Recognized										$ 20,800
Decoupling Mechanism [Member] \| Subsidiaries [Member] \| Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage				2.20%
Decoupling Mechanism [Member] \| Subsidiaries [Member] \| Electric [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage				3.00%
Decoupling Mechanism [Member] \| Maximum [Member] \| Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage		5.00%
Decoupling Mechanism [Member] \| Maximum [Member] \| Subsidiaries [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage		3.00%
Decoupling Mechanism [Member] \| Maximum [Member] \| Subsidiaries [Member] \| Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage		3.00%
General Rate Case [Member] \| Subsidiaries [Member]
Regulatory Assets [Line Items]
Number of new mechanisms			2
Regulated Utility, Allowed Rate of Return on Net Regulatory Assets and Liabilities		7.60%
Regulated Utility, After-tax Allowed Rate of Return on Net Regulatory Assets and Liabilities		6.55%
Public Utilities, Requested Equity Capital Structure, Percentage		48.50%
Public Utilities, Requested Return on Equity, Percentage		9.50%
General Rate Case [Member] \| Subsidiaries [Member] \| Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount		$ (35,500)
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage		(3.80%)
General Rate Case [Member] \| Subsidiaries [Member] \| Electric [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount		$ 20,200
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage		0.90%
Tax Cuts and Jobs Act [Member] \| Subsidiaries [Member] \| Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount	$ (23,600)
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage	(2.70%)
Tax Cuts and Jobs Act [Member] \| Subsidiaries [Member] \| Electric [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount	$ (72,900)
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage	(3.40%)
Colstrip Regulatory Asset [Domain]
Regulatory Assets [Line Items]
Regulatory Assets					$ 129,200			$ 129,200				$ 127,600