Regulation and Rates (Details) - USD ($)																						3 Months Ended		4 Months Ended	12 Months Ended				36 Months Ended
	Apr. 12, 2021	Apr. 02, 2021	Feb. 02, 2021	Dec. 09, 2020	Nov. 01, 2020	Sep. 30, 2020	Aug. 31, 2020	Aug. 30, 2020	Jul. 31, 2020	Jul. 08, 2020	Apr. 06, 2020	Jan. 15, 2020	Nov. 01, 2019	Oct. 31, 2019	Jun. 20, 2019	Apr. 10, 2019	Mar. 01, 2019	Jan. 30, 2019	Nov. 07, 2018	Dec. 19, 2017	Dec. 18, 2017	Mar. 31, 2021	Mar. 31, 2020	Apr. 30, 2018	Jul. 19, 2021	Dec. 31, 2020	May 01, 2020	Apr. 30, 2019	Jul. 19, 2023	Aug. 27, 2020	Jul. 21, 2020	Apr. 30, 2020	Dec. 31, 2019	Jan. 01, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Income tax (benefit) expense																						$ 2,153,000	$ 3,984,000
Depreciation & Amortization																						208,431,000	164,816,000
Other Regulatory Assets																						737,377,000				$ 747,651,000
Subsidiaries [Member]
Regulatory Assets [Line Items]
Regulated Utility, Allowed Rate of Return on Net Regulatory Assets and Liabilities																			7.49%
Income tax (benefit) expense																						16,626,000	13,265,000	$ 34,600,000
Annual Power Cost Variability, Interest																						300,000	500,000
PGA payable																						46,787,000				87,655,000							$ 132,766,000
Depreciation & Amortization																						208,362,000	164,771,000
Liabilities, Other than Long-term Debt, Noncurrent											$ 11,000,000.0																				$ 4,500,000
Customer Bill Assistance											$ 1,000
Other Regulatory Assets																						737,377,000				747,651,000
Subsidiaries [Member] | Subsequent Event [Member]
Regulatory Assets [Line Items]
Customer Bill Assistance	$ 2,500
Subsidiaries [Member] | Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Storm Damage Costs Incurred During Period																						23,300,000	9,900,000
Public Utilities, Rate Case, Deferred Storm Costs Threshold																																		$ 10,000,000.0	$ 8,000,000.0
Public Utilities, Rate Case, Deferred Storm Qualifying Costs																																		$ 500,000
Subsidiaries [Member] | Electricity | Subsequent Event [Member]
Regulatory Assets [Line Items]
Liabilities, Other than Long-term Debt, Noncurrent	20,000,000.0
Subsidiaries [Member] | Natural Gas | Subsequent Event [Member]
Regulatory Assets [Line Items]
Liabilities, Other than Long-term Debt, Noncurrent	$ 7,700,000
General Rate Case [Member] | Subsidiaries [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Return on Equity, Percentage												9.50%			7.62%
Regulated Utility, Allowed Rate of Return on Net Regulatory Assets and Liabilities												7.48%							7.49%	7.60%
Annual Revenue Requirement						$ 25,600,000
Protected Excess Deferred Income Tax																														$ 70,800,000
Fiscal Period Duration															4 years
Unprotected Excess Deferred Income Tax																														$ 38,900,000
General Rate Case [Member] | Subsidiaries [Member] | Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage															6.90%
Public Utilities, Requested Rate Increase (Decrease), Amount												$ 1,500,000
Public Utilities, Approved Rate Increase (Decrease), Amount									$ 27,700,000	$ 900,000
Public Utilities, Approved Rate Increase (Decrease), Percentage									1.30%	0.05%
Public Utilities, Approved Debt Capital Structure, Percentage										7.39%
Public Utilities, Interim Rate Increase (Decrease), Amount									$ 59,600,000	$ 29,500,000
Public Utilities, Interim Rate Increase (Decrease), Percentage									2.90%	1.60%
Public Utilities Attrition Adjustment Increase Decrease Amount										$ 23,900,000
PublicUtilitiesIncremenalRateIncreaseDecreaseAmount									$ 30,100,000
General Rate Case [Member] | Subsidiaries [Member] | Natural Gas, US Regulated [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage															7.90%
Public Utilities, Approved Rate Increase (Decrease), Amount									$ 200,000	$ 1,300,000
Public Utilities, Approved Rate Increase (Decrease), Percentage									0.02%	0.15%
Public Utilities, Interim Rate Increase (Decrease), Amount									$ 42,900,000	$ 36,500,000
Public Utilities, Interim Rate Increase (Decrease), Percentage									5.60%	4.00%
Public Utilities Attrition Adjustment Increase Decrease Amount										$ 16,200,000
PublicUtilitiesIncremenalRateIncreaseDecreaseAmount									$ 6,400,000
General Rate Case [Member] | Maximum [Member] | Subsidiaries [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Return on Equity, Percentage															9.80%
Public Utilities, Approved Debt Capital Structure, Net of Tax, Percentage										6.80%
Public Utilities, Approved Equity Capital Structure, Percentage										48.50%
Public Utilities, Approved Return on Equity, Percentage										9.40%
Expedited Rate Filing (ERF) [Member] | Subsidiaries [Member]
Regulatory Assets [Line Items]
Fiscal Period Duration																											1 year
Expedited Rate Filing (ERF) [Member] | Subsidiaries [Member] | Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage																			0.90%
Public Utilities, Requested Rate Increase (Decrease), Amount																			$ 18,900,000
Expedited Rate Filing (ERF) [Member] | Subsidiaries [Member] | Natural Gas, US Regulated [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage																			2.70%
Public Utilities, Requested Rate Increase (Decrease), Amount																	$ 6,100,000		$ 21,700,000
Public Utilities, Approved Rate Increase (Decrease), Amount																	$ 25,900,000	$ 21,500,000
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%)
Contract with Customer, Refund Liability																																$ 100,000
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%)
Decoupling Mechanism [Member]
Regulatory Assets [Line Items]
Deferred Revenue, Revenue Recognized																						0
Decoupling Mechanism [Member] | Subsidiaries [Member]
Regulatory Assets [Line Items]
Fiscal Period Duration							2 years	1 year
Amortization							$ 16,000,000.0
Decoupling Mechanism [Member] | Subsidiaries [Member] | Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Contract with Customer, Liability, Revenue Recognized																						900,000
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%
Purchased Gas Adjustment [Member] | Subsidiaries [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount					$ 32,600,000								$ 17,800,000
Purchased natural gas costs																						113,175,000				314,792,000
Purchased natural gas costs, recoverable																						(154,592,000)				(363,886,000)
Purchased natural gas adjustment, interest																						549,000				3,983,000
Annual revenue					37,400,000								100,600,000
Out of Cycle PGA					$ 69,400,000								54,000,000.0
Under collected commodity balances													114,400,000
Commodity Costs													10,800,000
Refund to Customers, remaining balance													4,100,000
Refund to Customers													$ 54,700,000
Fiscal Period Duration							3 years						2 years	1 year
Other Regulatory Assets																											$ 4,900,000
Get to Zero Deferral Filing [Member] | Subsidiaries [Member]
Regulatory Assets [Line Items]
Regulated Utility, Allowed Rate of Return on Net Regulatory Assets and Liabilities																6.89%
Depreciation & Amortization																						4,600,000				$ 2,800,000
Public Utilities, Property, Plant and Equipment, Equipment, Useful Life																10 years
Power Cost Only Rate Case | Subsidiaries [Member] | Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage			4.10%	3.70%
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 88,000,000.0	$ 78,500,000
Power Cost Only Rate Case | Subsidiaries [Member] | Electricity, US Regulated [Member] | Subsequent Event [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage		3.10%
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 65,300,000
Storm That Occurred In 2018 [Member] | Subsidiaries [Member] | Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Storm Damage Costs Deferred During Period																							$ 0
Storm That Occurred In 2019 [Member] | Subsidiaries [Member] | Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Storm Damage Costs Deferred During Period																						200,000
storm that occurred in 2020 [Member] | Subsidiaries [Member] | Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Storm Damage Costs Deferred During Period																						$ 12,900,000
Forecast [Member] | General Rate Case [Member] | Subsidiaries [Member]
Regulatory Assets [Line Items]
Fiscal Period Duration																									12 months				36 months