CONDENSED CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Statement [Abstract]
OPERATING REVENUES (Note 2)	$ 1,637,759	$ 1,469,871	$ 3,704,417	$ 3,315,071
OPERATING EXPENSES
Fuel and purchased power	614,520	556,571	1,416,778	1,174,027
Operations and maintenance	250,019	251,663	777,337	715,392
Depreciation and amortization	203,438	190,389	590,445	563,491
Taxes other than income taxes	53,169	53,475	167,949	165,591
Other expenses	350	200	1,648	1,410
Total	1,121,496	1,052,298	2,954,157	2,619,911
OPERATING INCOME	516,263	417,573	750,260	695,160
OTHER INCOME (DEDUCTIONS)
Allowance for equity funds used during construction	11,976	9,133	40,071	30,966
Pension and other postretirement non-service credits - net (Note 5)	10,174	24,673	30,513	73,739
Other income (Note 9)	15,941	2,219	28,424	5,605
Other expense (Note 9)	(6,972)	(6,745)	(15,916)	(14,751)
Total	31,119	29,280	83,092	95,559
INTEREST EXPENSE
Interest charges	96,909	72,185	278,860	205,677
Allowance for borrowed funds used during construction	(9,092)	(8,692)	(34,131)	(19,047)
Total	87,817	63,493	244,729	186,630
INCOME BEFORE INCOME TAXES	459,565	383,360	588,623	604,089
INCOME TAXES	57,045	52,728	74,125	83,577
NET INCOME	402,520	330,632	514,498	520,512
Less: Net income attributable to noncontrolling interests (Note 6)	4,306	4,306	12,918	12,918
NET INCOME ATTRIBUTABLE TO COMMON SHAREHOLDER	$ 398,214	$ 326,326	$ 501,580	$ 507,594
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — BASIC (in shares)	113,464	113,211	113,411	113,162
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — DILUTED (in shares)	113,838	113,463	113,718	113,376
EARNINGS PER WEIGHTED-AVERAGE COMMON SHARE OUTSTANDING
Net income attributable to common shareholders - basic (in dollars per share)	$ 3.51	$ 2.88	$ 4.42	$ 4.49
Net income attributable to common shareholders — diluted (in dollars per share)	$ 3.50	$ 2.88	$ 4.41	$ 4.48