CONDENSED CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Income Statement [Abstract]
OPERATING REVENUES	$ 1,469,871	$ 1,308,254	$ 3,315,071	$ 3,004,978
OPERATING EXPENSES
Fuel and purchased power	556,571	427,452	1,174,027	895,514
Operations and maintenance	251,663	232,386	715,392	692,131
Depreciation and amortization	190,389	163,523	563,491	480,093
Taxes other than income taxes	53,475	57,608	165,591	176,586
Other expenses - net	200	(2,088)	1,410	5,361
Total	1,052,298	878,881	2,619,911	2,249,685
OPERATING INCOME	417,573	429,373	695,160	755,293
OTHER INCOME (DEDUCTIONS)
Allowance for equity funds used during construction	9,133	11,352	30,966	30,549
Pension and other postretirement non-service credits - net (Note 5)	24,673	28,135	73,739	84,101
Other income (Note 9)	2,219	12,083	5,605	36,719
Other expense (Note 9)	(6,745)	(6,182)	(14,751)	(15,219)
Total	29,280	45,388	95,559	136,150
INTEREST EXPENSE
Interest charges	72,185	64,067	205,677	188,782
Allowance for borrowed funds used during construction	(8,692)	(5,273)	(19,047)	(15,466)
Total	63,493	58,794	186,630	173,316
INCOME BEFORE INCOME TAXES	383,360	415,967	604,089	718,127
INCOME TAXES	52,728	71,863	83,577	114,073
NET INCOME	330,632	344,104	520,512	604,054
Less: Comprehensive income attributable to noncontrolling interests	4,306	4,306	12,918	12,918
NET INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS	$ 326,326	$ 339,798	$ 507,594	$ 591,136
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — BASIC (in shares)	113,211	112,923	113,162	112,878
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — DILUTED (in shares)	113,463	113,217	113,376	113,178
EARNINGS PER WEIGHTED-AVERAGE COMMON SHARE OUTSTANDING
Net income attributable to common shareholders - basic (in dollars per share)	$ 2.88	$ 3.01	$ 4.49	$ 5.24
Net income attributable to common shareholders — diluted (in dollars per share)	$ 2.88	$ 3.00	$ 4.48	$ 5.22