CONDENSED CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Income Statement [Abstract]
OPERATING REVENUES (Note 2)	$ 1,121,703	$ 1,061,669	$ 2,066,658	$ 1,845,200
OPERATING EXPENSES
Fuel and purchased power	407,754	352,187	802,258	617,456
Operations and maintenance	277,238	245,387	527,318	463,729
Depreciation and amortization	195,101	186,497	387,007	373,102
Taxes other than income taxes	57,642	54,118	114,780	112,116
Other expenses	688	385	1,298	1,210
Total	938,423	838,574	1,832,661	1,567,613
OPERATING INCOME	183,280	223,095	233,997	277,587
OTHER INCOME (DEDUCTIONS)
Allowance for equity funds used during construction	13,034	12,086	28,095	21,833
Pension and other postretirement non-service credits - net (Note 5)	10,474	25,257	20,339	49,066
Other income (Note 9)	6,406	1,682	12,483	3,386
Other expense (Note 9)	(4,813)	(4,584)	(8,944)	(8,006)
Total	25,101	34,441	51,973	66,279
INTEREST EXPENSE
Interest charges	93,832	68,103	181,951	133,492
Allowance for borrowed funds used during construction	(12,317)	(5,873)	(25,039)	(10,355)
Total	81,515	62,230	156,912	123,137
INCOME BEFORE INCOME TAXES	126,866	195,306	129,058	220,729
INCOME TAXES	15,897	26,688	17,080	30,849
NET INCOME	110,969	168,618	111,978	189,880
Less: Comprehensive income attributable to noncontrolling interests	4,306	4,306	8,612	8,612
NET INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS	$ 106,663	$ 164,312	$ 103,366	$ 181,268
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — BASIC (in shares)	113,411	113,172	113,385	113,137
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — DILUTED (in shares)	113,717	113,369	113,657	113,332
EARNINGS PER WEIGHTED-AVERAGE COMMON SHARE OUTSTANDING
Net income attributable to common shareholders - basic (in dollars per share)	$ 0.94	$ 1.45	$ 0.91	$ 1.60
Net income attributable to common shareholders — diluted (in dollars per share)	$ 0.94	$ 1.45	$ 0.91	$ 1.60