CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Income Statement [Abstract]
OPERATING REVENUES (Note 2)	$ 5,124,915	$ 4,695,991	$ 4,324,385
OPERATING EXPENSES
Fuel and purchased power	1,822,566	1,792,657	1,629,343
Operations and maintenance	1,165,156	1,058,725	987,072
Depreciation and amortization	895,346	794,043	753,195
Taxes other than income taxes	227,395	224,013	220,370
Other expense	2,389	1,913	2,494
Total	4,112,852	3,871,351	3,592,474
OPERATING INCOME	1,012,063	824,640	731,911
OTHER INCOME (DEDUCTIONS)
Allowance for equity funds used during construction (Note 1)	38,620	53,118	45,263
Pension and other postretirement non-service credits, net (Note 7)	48,870	40,648	98,487
Other income (Note 16)	48,614	33,666	7,916
Other expense (Note 16)	(34,136)	(25,056)	(52,385)
Total	101,968	102,376	99,281
INTEREST EXPENSE
Interest charges	425,742	374,887	283,569
Allowance for borrowed funds used during construction (Note 1)	(48,270)	(43,564)	(28,030)
Total	377,472	331,323	255,539
INCOME BEFORE INCOME TAXES	736,559	595,693	575,653
INCOME TAXES (Note 4)	110,529	76,912	74,827
NET INCOME	626,030	518,781	500,826
Less: Net income attributable to noncontrolling interests (Note 17)	17,224	17,224	17,224
NET INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS	$ 608,806	$ 501,557	$ 483,602
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — BASIC (in shares)	113,846	113,442	113,196
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — DILUTED (in shares)	116,232	113,804	113,416
EARNINGS PER WEIGHTED-AVERAGE COMMON SHARE OUTSTANDING
Net income attributable to common shareholders — basic (in dollars per share)	$ 5.35	$ 4.42	$ 4.27
Net income attributable to common shareholders — diluted (in dollars per share)	$ 5.24	$ 4.41	$ 4.26