CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Utility Plant [Abstract]
Common plant	$ 1,115,852	$ 1,155,567
Less: Accumulated depreciation and amortization	(4,278,241)	(4,031,458)
Net utility plant	11,647,502	11,351,950
Other property and investments:
Goodwill	1,656,513	1,656,513
Other property and investments	326,306	324,897
Total other property and investments	1,982,819	1,981,410
Current assets:
Cash and cash equivalents	49,304	56,946
Restricted cash	12,436	46,204
Accounts receivable, net of allowance for doubtful accounts of $39,332 and $34,958, respectively	388,638	398,895
Unbilled revenue	158,591	271,606
Materials and supplies, at average cost	127,290	113,287
Fuel and natural gas inventory, at average cost	104,812	59,393
Unrealized gain on derivative instruments	212,583	128,210
Prepaid expense and other	52,945	46,293
Power contract acquisition adjustment gain	17,185	17,274
Total current assets	1,123,784	1,138,108
Other long-term and regulatory assets:
Power cost adjustment mechanism	49,029	79,546
Purchased gas adjustment receivable	49,426	57,935
Regulatory assets related to power contracts	8,179	9,689
Other regulatory assets	789,147	815,058
Unrealized gain on derivative instruments	65,515	26,197
Power contract acquisition adjustment gain	50,157	63,660
Operating lease right-of-use asset	176,568	184,957
Other	192,947	163,374
Total other long-term and regulatory assets	1,380,968	1,400,416
Total assets	16,135,073	15,871,884
Common shareholder’s equity:
Common stock $0.01 par value, 1,000 shares authorized, 200 shares outstanding	0	0
Additional paid-in capital	3,523,532	3,523,532
Retained earnings	1,217,063	1,067,216
Accumulated other comprehensive income (loss), net of tax	(20,874)	(27,432)
Total common shareholder’s equity	4,719,721	4,563,316
Long-term debt:
First mortgage bonds and senior notes	4,662,000	4,662,000
Pollution control bonds	161,860	161,860
Long-term debt	2,034,300	1,583,300
Debt discount issuance costs and other	(197,861)	(203,394)
Total long-term debt	6,660,299	6,203,766
Total capitalization	11,380,020	10,767,082
Current liabilities:
Accounts payable	408,915	444,384
Short-term debt	161,000	140,000
Current maturities of long-term debt	0	450,000
Accrued expenses:
Taxes	114,965	127,398
Salaries and wages	52,829	47,936
Interest	76,839	67,807
Unrealized loss on derivative instruments	71,213	63,309
Power contract acquisition adjustment loss	1,661	1,785
Operating lease liabilities	21,565	20,398
Other	69,490	62,406
Total current liabilities	978,477	1,425,423
Other long-term and regulatory liabilities:
Deferred income taxes	933,421	912,484
Unrealized loss on derivative instruments	49,798	40,965
Regulatory liabilities	948,453	844,184
Regulatory liability for deferred income taxes	829,005	865,976
Regulatory liabilities related to power contracts	67,342	80,934
Power contract acquisition adjustment loss	6,518	7,904
Operating lease liabilities	162,694	172,510
Finance lease liabilities	103,314	105,303
Other deferred credits	676,031	649,119
Total long-term and regulatory liabilities	3,776,576	3,679,379
Commitments and contingencies (Note 8)
Total capitalization and liabilities	16,135,073	15,871,884
Subsidiaries [Member]
Utility Plant [Abstract]
Common plant	1,137,473	1,177,598
Less: Accumulated depreciation and amortization	(6,634,784)	(6,416,246)
Net utility plant	11,647,502	11,351,950
Other property and investments:
Other property and investments	79,003	74,602
Total other property and investments	79,003	74,602
Current assets:
Cash and cash equivalents	44,759	50,043
Restricted cash	12,436	46,204
Accounts receivable, net of allowance for doubtful accounts of $39,332 and $34,958, respectively	389,694	402,602
Unbilled revenue	158,054	271,606
Materials and supplies, at average cost	127,290	113,287
Fuel and natural gas inventory, at average cost	102,918	58,129
Unrealized gain on derivative instruments	212,583	128,210
Prepaid expense and other	52,945	46,293
Total current assets	1,100,679	1,116,374
Other long-term and regulatory assets:
Power cost adjustment mechanism	49,029	79,546
Purchased gas adjustment receivable	49,426	57,935
Other regulatory assets	789,147	815,058
Unrealized gain on derivative instruments	65,515	26,197
Operating lease right-of-use asset	176,568	184,957
Other	189,403	162,391
Total other long-term and regulatory assets	1,319,088	1,326,084
Total assets	14,146,272	13,869,010
Common shareholder’s equity:
Common stock $0.01 par value, 1,000 shares authorized, 200 shares outstanding	859	859
Additional paid-in capital	3,535,105	3,485,105
Retained earnings	1,176,125	982,607
Accumulated other comprehensive income (loss), net of tax	(101,037)	(113,141)
Total common shareholder’s equity	4,611,052	4,355,430
Long-term debt:
First mortgage bonds and senior notes	4,662,000	4,662,000
Pollution control bonds	161,860	161,860
Debt discount issuance costs and other	(37,585)	(39,141)
Total long-term debt	4,786,275	4,784,719
Total capitalization	9,397,327	9,140,149
Current liabilities:
Accounts payable	408,829	451,716
Short-term debt	102,000	140,000
Accrued expenses:
Taxes	115,663	133,406
Salaries and wages	52,829	47,936
Interest	58,395	51,832
Unrealized loss on derivative instruments	71,213	63,309
Operating lease liabilities	21,565	20,398
Other	69,490	62,406
Total current liabilities	899,984	971,003
Other long-term and regulatory liabilities:
Deferred income taxes	1,084,184	1,084,203
Unrealized loss on derivative instruments	49,798	40,965
Regulatory liabilities	947,189	842,920
Regulatory liability for deferred income taxes	829,561	866,541
Operating lease liabilities	162,694	172,510
Finance lease liabilities	103,314	105,303
Other deferred credits	672,221	645,416
Total long-term and regulatory liabilities	3,848,961	3,757,858
Commitments and contingencies (Note 8)
Total capitalization and liabilities	14,146,272	13,869,010
Electricity, US Regulated [Member]
Utility Plant [Abstract]
Public Utility Property	10,150,577	9,729,643
Electricity, US Regulated [Member] | Subsidiaries [Member]
Utility Plant [Abstract]
Public Utility Property	11,930,798	11,535,976
Natural Gas, US Regulated [Member]
Utility Plant [Abstract]
Public Utility Property	4,659,314	4,498,198
Natural Gas, US Regulated [Member] | Subsidiaries [Member]
Utility Plant [Abstract]
Public Utility Property	$ 5,214,015	$ 5,054,622