CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Utility Plant [Abstract]
Electric plant	$ 9,729,643	$ 9,200,231
Natural gas plant	4,498,198	4,227,532
Common plant	1,155,567	1,116,524
Less: accumulated provision for depreciation	(4,031,458)	(3,671,094)
Net utility plant	11,351,950	10,873,193
Other property and investments:
Goodwill	1,656,513	1,656,513
Other property and investments	324,897	324,184
Total other property and investments	1,981,410	1,980,697
Current assets:
Cash and cash equivalents	56,946	52,307
Restricted cash	46,204	29,544
Accounts receivable, net of allowance for doubtful accounts	398,895	352,132
Unbilled revenue	271,606	221,871
Materials and supplies, at average cost	113,287	118,333
Fuel and natural gas inventory, at average cost	59,393	48,795
Unrealized gain on derivative instruments	128,210	33,015
Prepaid expenses and other	46,293	45,746
Power contract acquisition adjustment gain	17,274	14,874
Total current assets	1,138,108	916,617
Other long-term and regulatory assets:
Power cost adjustment mechanism	79,546	82,801
purchase gas adjustment, long-term	57,935	87,655
Regulatory assets related to power contracts	9,689	11,728
Other regulatory assets	815,058	747,651
Unrealized gain on derivative instruments	26,197	8,805
Operating lease right-of-use asset	184,957	172,167
Power contract acquisition adjustment gain	63,660	80,900
Other	163,374	80,751
Total other long-term and regulatory assets	1,400,416	1,272,458
Total assets	15,871,884	15,042,965
Common shareholder’s equity:
Common stock	0	0
Additional paid-in capital	3,523,532	3,313,532
Retained earnings	1,067,216	912,787
Accumulated other comprehensive income (loss), net of tax	(27,432)	(86,437)
Total common shareholder’s equity	4,563,316	4,139,882
Long-term debt:
First mortgage bonds and senior notes	4,662,000	4,212,000
Pollution control bonds	161,860	161,860
Long-term debt	1,583,300	1,724,700
Debt discount, issuance costs and other	(203,394)	(206,120)
Total long-term debt	6,203,766	5,892,440
Total capitalization	10,767,082	10,032,322
Current liabilities:
Accounts payable	444,384	342,404
Short-term debt	140,000	373,800
Current maturities of long-term debt	450,000	526,412
Accrued expenses:
Taxes	127,398	110,752
Salaries and wages	47,936	42,530
Interest	67,807	73,647
Unrealized loss on derivative instruments	63,309	31,441
Power contract acquisition adjustment loss	1,785	2,039
Operating lease liabilities	20,398	19,204
Other	62,406	73,385
Total current liabilities	1,425,423	1,595,614
Other Long-term and regulatory liabilities:
Deferred income taxes	912,484	810,729
Unrealized loss on derivative instruments	40,965	29,833
Regulatory liabilities	844,184	732,498
Regulatory liability for deferred income taxes	865,976	953,274
Regulatory liabilities related to power contracts	80,934	95,774
Power contract acquisition adjustment loss	7,904	9,689
Operating lease liabilities	172,510	160,980
Finance Lease, Liability, Noncurrent	105,303	320
Other deferred credits	649,119	621,932
Total long-term and regulatory liabilities	3,679,379	3,415,029
Commitments and contingencies (Note 16)
Total capitalization and liabilities	$ 15,871,884	$ 15,042,965
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	$ 11,535,976	$ 11,035,402
Natural gas plant	5,054,622	4,786,419
Common plant	1,177,598	1,139,120
Less: accumulated provision for depreciation	(6,416,246)	(6,087,748)
Net utility plant	11,351,950	10,873,193
Other property and investments:
Other property and investments	74,602	83,855
Total other property and investments	74,602	83,855
Current assets:
Cash and cash equivalents	50,043	51,177
Restricted cash	46,204	29,544
Accounts receivable, net of allowance for doubtful accounts	402,602	355,850
Unbilled revenue	271,606	221,871
Materials and supplies, at average cost	113,287	118,333
Fuel and natural gas inventory, at average cost	58,129	47,531
Unrealized gain on derivative instruments	128,210	33,015
Prepaid expenses and other	46,293	45,746
Total current assets	1,116,374	903,067
Other long-term and regulatory assets:
Power cost adjustment mechanism	79,546	82,801
purchase gas adjustment, long-term	57,935	87,655
Other regulatory assets	815,058	747,651
Unrealized gain on derivative instruments	26,197	8,805
Operating lease right-of-use asset	184,957	172,167
Other	162,391	79,231
Total other long-term and regulatory assets	1,326,084	1,178,310
Total assets	13,869,010	13,038,425
Common shareholder’s equity:
Common stock	859	859
Additional paid-in capital	3,485,105	3,485,105
Retained earnings	982,607	876,401
Accumulated other comprehensive income (loss), net of tax	(113,141)	(180,956)
Total common shareholder’s equity	4,355,430	4,181,409
Long-term debt:
First mortgage bonds and senior notes	4,662,000	4,212,000
Pollution control bonds	161,860	161,860
Debt discount, issuance costs and other	(39,141)	(35,816)
Total long-term debt	4,784,719	4,338,044
Total capitalization	9,140,149	8,519,453
Current liabilities:
Accounts payable	451,716	342,504
Short-term debt	140,000	373,800
Current maturities of long-term debt	0	2,412
Accrued expenses:
Taxes	133,406	107,254
Salaries and wages	47,936	42,530
Interest	51,832	48,189
Unrealized loss on derivative instruments	63,309	31,441
Operating lease liabilities	20,398	19,204
Other	62,406	73,385
Total current liabilities	971,003	1,040,719
Other Long-term and regulatory liabilities:
Deferred income taxes	1,084,203	987,382
Unrealized loss on derivative instruments	40,965	29,833
Regulatory liabilities	842,920	731,234
Regulatory liability for deferred income taxes	866,541	953,987
Operating lease liabilities	172,510	160,980
Finance Lease, Liability, Noncurrent	105,303	320
Other deferred credits	645,416	614,517
Total long-term and regulatory liabilities	3,757,858	3,478,253
Commitments and contingencies (Note 16)
Total capitalization and liabilities	$ 13,869,010	$ 13,038,425
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