CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Income Statement [Abstract]
OPERATING REVENUES (Note 2)	$ 4,324,385	$ 3,803,835	$ 3,586,982
OPERATING EXPENSES
Fuel and purchased power	1,629,343	1,152,551	993,419
Operations and maintenance	987,072	954,067	958,910
Depreciation and amortization	753,195	650,875	614,378
Taxes other than income taxes	220,370	234,639	224,835
Other expenses	2,494	6,393	7,288
Total	3,592,474	2,998,525	2,798,830
OPERATING INCOME	731,911	805,310	788,152
OTHER INCOME (DEDUCTIONS)
Allowance for equity funds used during construction (Note 1)	45,263	41,737	33,776
Pension and other postretirement non-service credits — net (Note 7)	98,487	112,541	56,341
Other income (Note 16)	7,916	45,100	56,703
Other expense (Note 16)	(52,385)	(25,396)	(57,776)
Total	99,281	173,982	89,044
INTEREST EXPENSE
Interest charges	283,569	254,314	247,501
Allowance for borrowed funds used during construction (Note 1)	(28,030)	(21,052)	(18,530)
Total	255,539	233,262	228,971
INCOME BEFORE INCOME TAXES	575,653	746,030	648,225
INCOME TAXES (Note 4)	74,827	110,086	78,173
NET INCOME	500,826	635,944	570,052
Less: Net income attributable to noncontrolling interests (Note 17)	17,224	17,224	19,493
NET INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS	$ 483,602	$ 618,720	$ 550,559
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — BASIC (in shares)	113,196	112,910	112,666
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — DILUTED (in shares)	113,416	113,192	112,942
EARNINGS PER WEIGHTED-AVERAGE COMMON SHARE OUTSTANDING
Net income attributable to common shareholders - basic (in dollars per share)	$ 4.27	$ 5.48	$ 4.89
Net income attributable to common shareholders — diluted (in dollars per share)	$ 4.26	$ 5.47	$ 4.87