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Regulation and Rates (Details)					1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
Regulation and Rates (Details)	May 01, 2018 USD ($)	Jan. 01, 2018 USD ($)	Dec. 19, 2017 USD ($)	Dec. 18, 2017	Jan. 31, 2017	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Apr. 30, 2019 USD ($)	Dec. 31, 2017 USD ($)
Regulatory Assets [Line Items]
Income tax (benefit) expense						$ (2,844,000)	$ 4,824,000	$ 12,428,000	$ 80,489,000
Subsidiaries [Member]
Regulatory Assets [Line Items]
Income tax (benefit) expense	$ 34,600,000					(1,765,000)	14,424,000	26,931,000	109,015,000
Subsidiaries [Member] \| Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Storm Damage Costs Incurred During Period						8,900,000	21,100,000
Storm Damage Costs Deferred During Period						0	$ 12,400,000
Public Utilities, Rate Case, Deferred Storm Costs Threshold		$ 10,000,000									$ 8,000,000
Public Utilities, Rate Case, Deferred Storm Qualifying Costs		$ 500,000
Decoupling Mechanism [Member] \| Subsidiaries [Member]
Regulatory Assets [Line Items]
Deferred Revenue, Revenue Recognized									$ 20,800,000
Decoupling Mechanism [Member] \| Subsidiaries [Member] \| Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage		3.00%
Deferred Revenue, Revenue Recognized								300,000
Decoupling Mechanism [Member] \| Subsidiaries [Member] \| Natural Gas, US Regulated [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage		2.20%
Decoupling Mechanism [Member] \| Maximum [Member] \| Subsidiaries [Member] \| Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage			3.00%
Decoupling Mechanism [Member] \| Maximum [Member] \| Subsidiaries [Member] \| Natural Gas, US Regulated [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage			5.00%	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] \| Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount			$ 20,200,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage			0.90%
General Rate Case [Member] \| Subsidiaries [Member] \| Natural Gas, US Regulated [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount			$ (35,500,000)
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage			(3.80%)
Scenario, Forecast [Member] \| Tax Cuts and Jobs Act [Member] \| Subsidiaries [Member] \| Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount										$ (72,900,000)
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage										(3.40%)
Scenario, Forecast [Member] \| Tax Cuts and Jobs Act [Member] \| Subsidiaries [Member] \| Natural Gas, US Regulated [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount										$ (23,600,000)
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage										(2.70%)
Colstrip Regulatory Asset [Domain]
Regulatory Assets [Line Items]
Regulatory Assets						$ 129,900,000		$ 129,900,000			$ 127,600,000