CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Operating Revenues	$ 1,432,618	$ 1,340,935	$ 1,234,995
Operating Expenses
Fuel	351,749	285,551	289,862
Purchased Power	134,914	136,425	85,354
Transmission and Other PPFAC Recoverable Costs	46,595	36,239	23,781
Increase (Decrease) to Reflect PPFAC Recovery Treatment	9,885	(32,660)	21,064
Total Fuel and Purchased Power	543,143	425,555	420,061
Operations and Maintenance	361,963	360,302	353,905
Depreciation	158,310	152,874	146,097
Amortization	26,052	22,255	22,498
Taxes Other Than Income Taxes	55,006	53,623	49,303
Total Operating Expenses	1,144,474	1,014,609	991,864
Operating Income	288,144	326,326	243,131
Other Income (Expense)
Interest Expense	(67,620)	(65,290)	(65,902)
Allowance For Borrowed Funds	3,151	2,078	1,710
Allowance For Equity Funds	8,117	5,322	4,522
Other, Net	(487)	8,995	353
Total Other Income (Expense)	(56,839)	(48,895)	(59,317)
Income Before Income Tax Expense	231,305	277,431	183,814
Income Tax Expense	42,982	100,763	59,376
Net Income	$ 188,323	$ 176,668	$ 124,438