Regulation and Rates (Details) - USD ($)																	3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Dec. 01, 2021	Nov. 01, 2021	Oct. 15, 2021	Oct. 01, 2021	Sep. 28, 2021	Jul. 01, 2021	Apr. 12, 2021	Mar. 02, 2021	Feb. 02, 2021	Dec. 09, 2020	Oct. 07, 2020	Jul. 31, 2020	Jul. 08, 2020	Apr. 13, 2020	Jun. 20, 2019	Apr. 10, 2019	Sep. 30, 2021	Sep. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Jul. 21, 2020	Apr. 06, 2020	Dec. 31, 2019	Jan. 01, 2018
Regulatory Assets [Line Items]
Income tax (benefit) expense																	$ 18,462,000	$ (559,000)			$ 32,946,000	$ 2,237,000
Depreciation & Amortization																	162,743,000	161,209,000			537,104,000	462,890,000
Other Regulatory Assets																	814,015,000				814,015,000		$ 747,651,000
Revenues																	770,236,000	620,426,000			2,699,712,000	2,318,235,000
Subsidiaries [Member]
Regulatory Assets [Line Items]
Income tax (benefit) expense																	18,902,000	1,649,000			49,953,000	16,913,000
Annual Power Cost Variability, Interest																					1,200,000	1,600,000
PGA payable																	56,268,000				56,268,000		87,655,000			$ 132,766,000
Depreciation & Amortization																	162,629,000	161,155,000			536,794,000	462,742,000
Liabilities, Other than Long-term Debt, Noncurrent							$ 27,700,000											8,900,000				8,900,000		$ 4,500,000	$ 11,000,000
Customer Bill Assistance							2,500							$ 1,000
Other Regulatory Assets																	814,015,000				814,015,000		747,651,000
Revenues																	770,236,000	$ 620,426,000			2,699,712,000	$ 2,318,235,000
Number of Households																		15,000				15,000
Subsidiaries [Member] | Under-collection [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount																					49,700,000	$ 51,500,000
Subsidiaries [Member] | Under-collection [Member] | Customer's share [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount																					20,300,000	21,900,000
Subsidiaries [Member] | Maximum Power | Customer's share plus interest
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount																			$ 20,000,000
Subsidiaries [Member] | Subsequent Event [Member]
Regulatory Assets [Line Items]
Liabilities, Other than Long-term Debt, Noncurrent			$ 34,500,000
Customer Bill Assistance			2,500
Subsidiaries [Member] | Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Storm Damage Costs Incurred During Period																					29,000,000	15,500,000
Public Utilities, Rate Case, Deferred Storm Costs Threshold																											$ 10,000,000
Public Utilities, Rate Case, Deferred Storm Qualifying Costs																											$ 500,000
Subsidiaries [Member] | Electricity
Regulatory Assets [Line Items]
Liabilities, Other than Long-term Debt, Noncurrent							20,000,000
Subsidiaries [Member] | Electricity | Subsequent Event [Member]
Regulatory Assets [Line Items]
Liabilities, Other than Long-term Debt, Noncurrent			23,700,000
Subsidiaries [Member] | Natural Gas
Regulatory Assets [Line Items]
Liabilities, Other than Long-term Debt, Noncurrent							$ 7,700,000
Subsidiaries [Member] | Natural Gas | Subsequent Event [Member]
Regulatory Assets [Line Items]
Liabilities, Other than Long-term Debt, Noncurrent			$ 10,800,000
General Rate Case [Member] | Subsidiaries [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount				$ 18,900,000
Annual Revenue Requirement											$ 25,600,000
Regulatory Assets																	$ 24,700,000				24,700,000
Revenues																					24,700,000
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, Approved Rate Increase (Decrease), Amount				$ 15,800,000									$ 900,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount					$ 77,100,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage					3.70%
Public Utilities, Approved Rate Increase (Decrease), Percentage				0.70%									0.05%
Public Utilities, Approved Debt Capital Structure, Percentage													7.39%
Public Utilities, Interim Rate Increase (Decrease), Amount					$ 48,300,000							$ 59,600,000	$ 29,500,000
Public Utilities, Interim Rate Increase (Decrease), Percentage					2.30%								1.60%
PublicUtilitiesIncremenalRateIncreaseDecreaseAmount					$ 17,500,000
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				$ 3,100,000									$ 1,300,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount					$ 45,300,000
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage					5.90%
Public Utilities, Approved Rate Increase (Decrease), Percentage				0.30%									0.15%
Public Utilities, Interim Rate Increase (Decrease), Amount					$ 4,900,000							$ 42,900,000	$ 36,500,000
Public Utilities, Interim Rate Increase (Decrease), Percentage					0.60%								4.00%
PublicUtilitiesIncremenalRateIncreaseDecreaseAmount					$ 2,400,000
General Rate Case [Member] | Maximum [Member] | Subsidiaries [Member]
Regulatory Assets [Line Items]
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%
Decoupling Mechanism [Member]
Regulatory Assets [Line Items]
Deferred Revenue, Revenue Recognized																					0
Decoupling Mechanism [Member] | Subsidiaries [Member] | Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Contract with Customer, Liability, Revenue Recognized																					2,000,000
Purchased Gas Adjustment [Member] | Subsidiaries [Member]
Regulatory Assets [Line Items]
Purchased natural gas costs																					218,934,000		314,792,000
Purchased natural gas costs, recoverable																					(251,633,000)		(363,886,000)
Purchased natural gas adjustment, interest																					1,312,000		$ 3,983,000
Purchased Gas Adjustment [Member] | Subsidiaries [Member] | Subsequent Event [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 59,100,000
Out of Cycle PGA		69,400,000
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																				$ 2,800,000	7,900,000
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								3.10%	4.10%	3.70%
Public Utilities, Requested Rate Increase (Decrease), Amount								$ 65,300,000	$ 88,000,000	$ 78,500,000
Public Utilities, Approved Rate Increase (Decrease), Amount						$ 70,900,000
Public Utilities, Approved Rate Increase (Decrease), Percentage						3.30%
Purchased Gas Adjustment, Schedule 101 | Subsidiaries [Member] | Subsequent Event [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		80,600,000
purchased gas adjustment, Schedule 106 106B | Subsidiaries [Member] | Subsequent Event [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 21,500,000
Storm That Occurred In 2018 [Member] | Subsidiaries [Member] | Electricity, US Regulated [Member]
Regulatory Assets [Line Items]
Storm Damage Costs Deferred During Period																						$ 5,300,000
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																					$ 19,000,000
Forecast [Member] | Subsidiaries [Member] | Under-collection [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount	$ 76,100,000
Forecast [Member] | Subsidiaries [Member] | Under-collection [Member] | Companys share [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount	32,100,000
Forecast [Member] | Subsidiaries [Member] | Under-collection [Member] | Customer's share [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount	44,000,000
Forecast [Member] | Subsidiaries [Member] | Under-collection [Member] | Customer's share plus interest
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount	$ 46,000,000