CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
OPERATING REVENUES (Note 2)	$ 3,803,835	$ 3,586,982	$ 3,471,209
OPERATING EXPENSES
Fuel and purchased power	1,152,551	993,419	1,042,237
Operations and maintenance	954,067	958,910	941,616
Depreciation and amortization	650,875	614,378	590,929
Taxes other than income taxes	234,639	224,835	218,579
Other expenses	6,393	7,288	5,888
Total	2,998,525	2,798,830	2,799,249
OPERATING INCOME	805,310	788,152	671,960
OTHER INCOME (DEDUCTIONS)
Allowance for equity funds used during construction (Note 1)	41,737	33,776	31,431
Pension and other postretirement non-service credits — net (Note 8)	112,541	56,341	22,989
Other income (Note 17)	45,100	56,703	50,263
Other expense (Note 17)	(25,396)	(57,776)	(17,880)
Total	173,982	89,044	86,803
INTEREST EXPENSE
Interest charges	254,314	247,501	235,251
Allowance for borrowed funds used during construction (Note 1)	(21,052)	(18,530)	(18,528)
Total	233,262	228,971	216,723
INCOME BEFORE INCOME TAXES	746,030	648,225	542,040
INCOME TAXES (Note 5)	110,086	78,173	(15,773)
NET INCOME	635,944	570,052	557,813
Less: Net income attributable to noncontrolling interests (Note 18)	17,224	19,493	19,493
NET INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS	$ 618,720	$ 550,559	$ 538,320
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — BASIC (in shares)	112,910	112,666	112,443
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — DILUTED (in shares)	113,192	112,942	112,758
EARNINGS PER WEIGHTED-AVERAGE COMMON SHARE OUTSTANDING
Net income attributable to common shareholders - basic (in dollars per share)	$ 5.48	$ 4.89	$ 4.79
Net income attributable to common shareholders — diluted (in dollars per share)	$ 5.47	$ 4.87	$ 4.77