CONDENSED CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Income Statement [Abstract]
OPERATING REVENUES (Note 2)	$ 1,061,669	$ 1,000,249	$ 1,845,200	$ 1,696,724
OPERATING EXPENSES
Fuel and purchased power	352,187	269,835	617,456	468,062
Operations and maintenance	245,387	229,690	463,729	459,745
Depreciation and amortization	186,497	158,750	373,102	316,570
Taxes other than income taxes	54,118	59,495	112,116	118,978
Other expenses	385	4,093	1,210	7,449
Total	838,574	721,863	1,567,613	1,370,804
OPERATING INCOME	223,095	278,386	277,587	325,920
OTHER INCOME (DEDUCTIONS)
Allowance for equity funds used during construction	12,086	9,990	21,833	19,197
Pension and other postretirement non-service credits - net (Note 5)	25,257	28,175	49,066	55,966
Other income (Note 9)	1,682	12,207	3,386	24,636
Other expense (Note 9)	(4,584)	(5,184)	(8,006)	(9,037)
Total	34,441	45,188	66,279	90,762
INTEREST EXPENSE
Interest charges	68,103	62,777	133,492	124,715
Allowance for borrowed funds used during construction	(5,873)	(5,199)	(10,355)	(10,193)
Total	62,230	57,578	123,137	114,522
INCOME BEFORE INCOME TAXES	195,306	265,996	220,729	302,160
INCOME TAXES	26,688	46,560	30,849	42,210
NET INCOME	168,618	219,436	189,880	259,950
Less: Comprehensive income attributable to noncontrolling interests	4,306	3,739	8,612	8,612
NET INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS	$ 164,312	$ 215,697	$ 181,268	$ 251,338
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — BASIC (in shares)	113,172	112,882	113,137	112,855
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — DILUTED (in shares)	113,369	113,223	113,332	113,158
EARNINGS PER WEIGHTED-AVERAGE COMMON SHARE OUTSTANDING
Net income attributable to common shareholders - basic (in dollars per share)	$ 1.45	$ 1.91	$ 1.60	$ 2.23
Net income attributable to common shareholders — diluted (in dollars per share)	$ 1.45	$ 1.91	$ 1.60	$ 2.22