CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Utility Plant [Abstract]
Electric plant	$ 10,300,895	$ 9,729,643
Natural gas plant	4,721,982	4,498,198
Common plant	1,103,783	1,155,567
Less: Accumulated depreciation and amortization	(4,341,789)	(4,031,458)
Net utility plant	11,784,871	11,351,950
Other property and investments:
Goodwill	1,656,513	1,656,513
Other property and investments	328,535	324,897
Total other property and investments	1,985,048	1,981,410
Current assets:
Cash and cash equivalents	105,740	56,946
Restricted cash	63,045	46,204
Accounts receivable, net of allowance for doubtful accounts of $41,962 and $34,958, respectively	673,236	398,895
Unbilled revenue	284,022	271,606
Materials and supplies, at average cost	132,172	113,287
Fuel and natural gas inventory, at average cost	94,075	59,393
Unrealized gain on derivative instruments	587,029	128,210
Prepaid expenses and other	41,940	46,293
Power contract acquisition adjustment gain	16,736	17,274
Total current assets	1,997,995	1,138,108
Other long-term and regulatory assets:
Power cost adjustment mechanism	112,207	79,546
Purchased gas adjustment receivable	0	57,935
Regulatory assets related to power contracts	7,904	9,689
Other regulatory assets	784,231	815,058
Unrealized gain on derivative instruments	94,621	26,197
Power contract acquisition adjustment gain	46,924	63,660
Operating lease right-of-use asset	193,509	184,957
Other	180,204	163,374
Total other long-term and regulatory assets	1,419,600	1,400,416
Total assets	17,187,514	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	1,465,331	1,067,216
Accumulated other comprehensive income (loss), net of tax	(24,774)	(27,432)
Total common shareholder’s equity	4,964,089	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	194,787	203,394
Total long-term debt	6,663,373	6,203,766
Total capitalization	11,627,462	10,767,082
Current liabilities:
Accounts payable	665,750	444,384
Short-term debt	441,300	140,000
Current maturities of long-term debt	0	450,000
Accrued expenses:
Taxes	116,098	127,398
Salaries and wages	60,537	47,936
Interest	62,148	67,807
Unrealized loss on derivative instruments	124,976	63,309
Power contract acquisition adjustment loss	1,638	1,785
Operating lease liabilities	20,342	20,398
Other	70,685	62,406
Total current liabilities	1,563,474	1,425,423
Other Long-term and regulatory liabilities:
Deferred income taxes	985,947	912,484
Unrealized loss on derivative instruments	18,366	40,965
Purchased gas adjustment liability	3,536	0
Regulatory liabilities	1,147,143	844,184
Regulatory liability for deferred income taxes	811,161	865,976
Regulatory liabilities related to power contracts	63,660	80,934
Power contract acquisition adjustment loss	6,266	7,904
Operating lease liabilities	181,265	172,510
Finance lease liabilities	102,518	105,303
Other deferred credits	676,716	649,119
Total long-term and regulatory liabilities	3,996,578	3,679,379
Commitments and contingencies (Note 16)
Total capitalization and liabilities	$ 17,187,514	$ 15,871,884
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	1,000	1,000
Common stock, shares outstanding (in shares)	200	200
Subsidiaries [Member]
Utility Plant [Abstract]
Electric plant	$ 12,071,531	$ 11,535,976
Natural gas plant	5,276,156	5,054,622
Common plant	1,125,217	1,177,598
Less: Accumulated depreciation and amortization	(6,688,033)	(6,416,246)
Net utility plant	11,784,871	11,351,950
Other property and investments:
Other property and investments	80,076	74,602
Total other property and investments	80,076	74,602
Current assets:
Cash and cash equivalents	102,840	50,043
Restricted cash	63,045	46,204
Accounts receivable, net of allowance for doubtful accounts of $41,962 and $34,958, respectively	671,071	402,602
Unbilled revenue	284,014	271,606
Materials and supplies, at average cost	132,172	113,287
Fuel and natural gas inventory, at average cost	91,783	58,129
Unrealized gain on derivative instruments	587,029	128,210
Prepaid expenses and other	41,940	46,293
Total current assets	1,973,894	1,116,374
Other long-term and regulatory assets:
Power cost adjustment mechanism	112,207	79,546
Purchased gas adjustment receivable	0	57,935
Other regulatory assets	784,231	815,058
Unrealized gain on derivative instruments	94,621	26,197
Operating lease right-of-use asset	193,509	184,957
Other	176,833	162,391
Total other long-term and regulatory assets	1,361,401	1,326,084
Total assets	15,200,242	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	1,438,163	982,607
Accumulated other comprehensive income (loss), net of tax	(103,044)	(113,141)
Total common shareholder’s equity	4,871,083	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	37,095	39,141
Total long-term debt	4,786,765	4,784,719
Total capitalization	9,657,848	9,140,149
Current liabilities:
Accounts payable	664,457	451,716
Short-term debt	357,000	140,000
Accrued expenses:
Taxes	116,472	133,406
Salaries and wages	60,537	47,936
Interest	52,170	51,832
Unrealized loss on derivative instruments	124,976	63,309
Operating lease liabilities	20,342	20,398
Other	70,685	62,406
Total current liabilities	1,466,639	971,003
Other Long-term and regulatory liabilities:
Deferred income taxes	1,139,600	1,084,203
Unrealized loss on derivative instruments	18,366	40,965
Purchased gas adjustment liability	3,536	0
Regulatory liabilities	1,145,879	842,920
Regulatory liability for deferred income taxes	811,724	866,541
Operating lease liabilities	181,265	172,510
Finance lease liabilities	102,518	105,303
Other deferred credits	672,867	645,416
Total long-term and regulatory liabilities	4,075,755	3,757,858
Commitments and contingencies (Note 16)
Total capitalization and liabilities	$ 15,200,242	$ 13,869,010
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	150,000,000	150,000,000
Common stock, shares outstanding (in shares)	85,903,791	85,903,791