CONDENSED CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Income Statement [Abstract]
OPERATING REVENUES (Note 2)	$ 783,531	$ 696,475
OPERATING EXPENSES
Fuel and purchased power	265,269	198,227
Operations and maintenance	218,342	230,055
Depreciation and amortization	186,605	157,820
Taxes other than income taxes	57,998	59,483
Other expenses	825	3,356
Total	729,039	648,941
OPERATING INCOME	54,492	47,534
OTHER INCOME (DEDUCTIONS)
Allowance for equity funds used during construction	9,747	9,207
Pension and other postretirement non-service credits - net	23,809	27,791
Other income (Note 9)	1,704	12,429
Other expense (Note 9)	(3,422)	(3,853)
Total	31,838	45,574
INTEREST EXPENSE
Interest charges	65,389	61,938
Allowance for borrowed funds used during construction	(4,482)	(4,994)
Total	60,907	56,944
INCOME BEFORE INCOME TAXES	25,423	36,164
INCOME TAXES	4,161	(4,350)
NET INCOME	21,262	40,514
Less: Comprehensive income attributable to noncontrolling interests	4,306	4,873
NET INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS	$ 16,956	$ 35,641
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — BASIC (in shares)	113,102	112,829
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING — DILUTED (in shares)	113,295	113,093
EARNINGS PER WEIGHTED-AVERAGE COMMON SHARE OUTSTANDING
Net income attributable to common shareholders - basic (in dollars per share)	$ 0.15	$ 0.32
Net income attributable to common shareholders — diluted (in dollars per share)	$ 0.15	$ 0.32