CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Operating Revenues	$ 1,592,586	$ 1,424,741	$ 1,418,338
Operating Expenses
Fuel	399,914	302,637	358,394
Purchased Power	204,264	146,968	137,977
Transmission and Other PPFAC Recoverable Costs	65,583	52,860	52,261
Increase (Decrease) to Reflect PPFAC Recovery Treatment	(64,155)	12,565	(42,836)
Total Fuel and Purchased Power	605,606	515,030	505,796
Operations and Maintenance	397,095	351,584	377,563
Depreciation	201,524	189,051	169,042
Amortization	43,995	28,754	27,706
Taxes Other Than Income Taxes	62,010	58,222	55,642
Total Operating Expenses	1,310,230	1,142,641	1,135,749
Operating Income	282,356	282,100	282,589
Other Income (Expense)
Interest Expense	(86,865)	(88,214)	(88,511)
Allowance For Borrowed Funds	6,624	9,480	5,744
Allowance For Equity Funds	17,885	22,847	15,222
Unrealized Gains on Investments	3,898	1,741	6,015
Other, Net	9,823	4,903	(491)
Total Other Income (Expense)	(48,635)	(49,243)	(62,021)
Income Before Income Tax Expense	233,721	232,857	220,568
Income Tax Expense	32,476	41,452	34,053
Net Income	$ 201,245	$ 191,405	$ 186,515