CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Statement [Abstract]
OPERATING REVENUES (Note 2)	$ 4,695,991	$ 4,324,385	$ 3,803,835
OPERATING EXPENSES
Fuel and purchased power	1,792,657	1,629,343	1,152,551
Operations and maintenance	1,058,725	987,072	954,067
Depreciation and amortization	794,043	753,195	650,875
Taxes other than income taxes	224,013	220,370	234,639
Other expenses	1,913	2,494	6,393
Total	3,871,351	3,592,474	2,998,525
OPERATING INCOME	824,640	731,911	805,310
OTHER INCOME (DEDUCTIONS)
Allowance for equity funds used during construction (Note 1)	53,118	45,263	41,737
Pension and other postretirement non-service credits, net (Note 7)	40,648	98,487	112,541
Other income (Note 16)	33,666	7,916	45,100
Other expense (Note 16)	(25,056)	(52,385)	(25,396)
Total	102,376	99,281	173,982
INTEREST EXPENSE
Interest charges	374,887	283,569	254,314
Allowance for borrowed funds used during construction (Note 1)	(43,564)	(28,030)	(21,052)
Total	331,323	255,539	233,262
INCOME BEFORE INCOME TAXES	595,693	575,653	746,030
INCOME TAXES (Note 4)	76,912	74,827	110,086
NET INCOME	518,781	500,826	635,944
Less: Net income attributable to noncontrolling interests (Note 17)	17,224	17,224	17,224
NET INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS	$ 501,557	$ 483,602	$ 618,720
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — BASIC (in shares)	113,442	113,196	112,910
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — DILUTED (in shares)	113,804	113,416	113,192
EARNINGS PER WEIGHTED-AVERAGE COMMON SHARE OUTSTANDING
Net income attributable to common shareholders - basic (in dollars per share)	$ 4.42	$ 4.27	$ 5.48
Net income attributable to common shareholders — diluted (in dollars per share)	$ 4.41	$ 4.26	$ 5.47