CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Utility Plant [Abstract]
Public Utilities Property Plant And Equipment Electric Plant	$ 10,431,708	$ 10,300,895
Public Utilities Property Plant And Equipment Gas Plant	4,778,660	4,721,982
Public Utilities Property Plant And Equipment Common Plant	1,098,813	1,103,783
Less: Accumulated depreciation and amortization	(4,448,520)	(4,341,789)
Net utility plant	11,860,661	11,784,871
Other property and investments:
Goodwill	1,656,513	1,656,513
Other property and investments	318,265	328,535
Total other property and investments	1,974,778	1,985,048
Current assets:
Cash and cash equivalents	110,974	105,740
Restricted cash	20,329	63,045
Accounts receivable, net of allowance for doubtful accounts of $44,228 and $41,962, respectively	603,944	673,236
Unbilled revenue	263,420	284,022
Materials and supplies, at average cost	136,745	132,172
Fuel and natural gas inventory, at average cost	56,562	94,075
Unrealized gain on derivative instruments	125,377	587,029
Prepaid expense and other	42,208	41,940
Power contract acquisition adjustment gain	16,807	16,736
Total current assets	1,376,366	1,997,995
Other long-term and regulatory assets:
Power cost adjustment mechanism	101,577	112,207
Regulatory assets related to power contracts	7,621	7,904
Other regulatory assets	827,659	784,231
Unrealized gain on derivative instruments	64,498	94,621
Power contract acquisition adjustment gain	42,525	46,924
Operating lease right-of-use asset	190,640	193,509
Other	196,464	180,204
Total other long-term and regulatory assets	1,430,984	1,419,600
Total assets	16,642,789	17,187,514
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	1,437,167	1,465,331
Accumulated other comprehensive income (loss), net of tax	(25,432)	(24,774)
Total common shareholder’s equity	4,935,267	4,964,089
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	2,034,300
Debt discount issuance costs and other	(191,744)	(194,787)
Total long-term debt	6,666,416	6,663,373
Total capitalization	11,601,683	11,627,462
Current liabilities:
Accounts payable	409,130	665,750
Short-term debt	237,700	441,300
Accrued expenses:
Taxes	200,050	116,098
Salaries and wages	47,616	60,537
Interest	77,202	62,148
Unrealized loss on derivative instruments	112,634	124,976
Power contract acquisition adjustment loss	1,586	1,638
Operating lease liabilities	20,254	20,342
Other	83,088	70,685
Total current liabilities	1,189,260	1,563,474
Other long-term and regulatory liabilities:
Deferred income taxes	928,320	985,947
Unrealized loss on derivative instruments	14,476	18,366
Purchased Gas Adjustment Liability, Long-term	102,914	3,536
Regulatory liabilities	890,059	1,147,143
Regulatory liability for deferred income taxes	797,697	811,161
Regulatory liabilities related to power contracts	59,332	63,660
Power contract acquisition adjustment loss	6,035	6,266
Operating lease liabilities	177,516	181,265
Finance lease liabilities	101,717	102,518
Compliance obligation	94,626	0
Other deferred credits	679,154	676,716
Total long-term and regulatory liabilities	3,851,846	3,996,578
Commitments and contingencies (Note 8)
Total capitalization and liabilities	16,642,789	17,187,514
Subsidiaries [Member]
Utility Plant [Abstract]
Public Utilities Property Plant And Equipment Electric Plant	12,194,280	12,071,531
Public Utilities Property Plant And Equipment Gas Plant	5,332,181	5,276,156
Public Utilities Property Plant And Equipment Common Plant	1,119,925	1,125,217
Less: Accumulated depreciation and amortization	(6,785,725)	(6,688,033)
Net utility plant	11,860,661	11,784,871
Other property and investments:
Other property and investments	71,388	80,076
Total other property and investments	71,388	80,076
Current assets:
Cash and cash equivalents	108,117	102,840
Restricted cash	20,329	63,045
Accounts receivable, net of allowance for doubtful accounts of $44,228 and $41,962, respectively	602,300	671,071
Unbilled revenue	263,420	284,014
Materials and supplies, at average cost	136,745	132,172
Fuel and natural gas inventory, at average cost	54,433	91,783
Unrealized gain on derivative instruments	125,377	587,029
Prepaid expense and other	42,070	41,940
Total current assets	1,352,791	1,973,894
Other long-term and regulatory assets:
Power cost adjustment mechanism	101,577	112,207
Other regulatory assets	827,659	784,231
Unrealized gain on derivative instruments	64,498	94,621
Operating lease right-of-use asset	190,640	193,509
Other	193,267	176,833
Total other long-term and regulatory assets	1,377,641	1,361,401
Total assets	14,662,481	15,200,242
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	3,535,105
Retained earnings	1,437,696	1,438,163
Accumulated other comprehensive income (loss), net of tax	(102,911)	(103,044)
Total common shareholder’s equity	4,870,749	4,871,083
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	(36,619)	(37,095)
Total long-term debt	4,787,241	4,786,765
Total capitalization	9,657,990	9,657,848
Current liabilities:
Accounts payable	408,802	664,457
Short-term debt	137,000	357,000
Accrued expenses:
Taxes	206,507	116,472
Salaries and wages	47,616	60,537
Interest	58,702	52,170
Unrealized loss on derivative instruments	112,634	124,976
Operating lease liabilities	20,254	20,342
Other	83,088	70,685
Total current liabilities	1,074,603	1,466,639
Other long-term and regulatory liabilities:
Deferred income taxes	1,076,317	1,139,600
Unrealized loss on derivative instruments	14,476	18,366
Purchased Gas Adjustment Liability, Long-term	102,914	3,536
Regulatory liabilities	888,795	1,145,879
Regulatory liability for deferred income taxes	798,257	811,724
Operating lease liabilities	177,516	181,265
Finance lease liabilities	101,717	102,518
Compliance obligation	94,626	0
Other deferred credits	675,270	672,867
Total long-term and regulatory liabilities	3,929,888	4,075,755
Commitments and contingencies (Note 8)
Total capitalization and liabilities	14,662,481	$ 15,200,242
Electricity, US Regulated [Member] | Subsidiaries [Member]
Other long-term and regulatory liabilities:
Compliance obligation	11,300
Natural Gas, US Regulated [Member] | Subsidiaries [Member]
Other long-term and regulatory liabilities:
Compliance obligation	$ 83,300