CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Operating Revenues	$ 1,418,338	$ 1,432,618	$ 1,340,935
Operating Expenses
Fuel	358,394	351,749	285,551
Purchased Power	137,977	134,914	136,425
Transmission and Other PPFAC Recoverable Costs	52,261	46,595	36,239
Increase (Decrease) to Reflect PPFAC Recovery Treatment	(42,836)	9,885	(32,660)
Total Fuel and Purchased Power	505,796	543,143	425,555
Operations and Maintenance	377,563	361,963	360,302
Depreciation	169,042	158,310	152,874
Amortization	27,706	26,052	22,255
Taxes Other Than Income Taxes	55,642	55,006	53,623
Total Operating Expenses	1,135,749	1,144,474	1,014,609
Operating Income	282,589	288,144	326,326
Other Income (Expense)
Interest Expense	(88,511)	(67,620)	(65,290)
Allowance For Borrowed Funds	5,744	3,151	2,078
Allowance For Equity Funds	15,222	8,117	5,322
Other, Net	5,524	(487)	8,995
Total Other Income (Expense)	(62,021)	(56,839)	(48,895)
Income Before Income Tax Expense	220,568	231,305	277,431
Income Tax Expense	34,053	42,982	100,763
Net Income	$ 186,515	$ 188,323	$ 176,668