CONDENSED CONSOLIDATED STATEMENTS OF INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Operating Revenues	$ 459,760	$ 417,210	$ 1,089,096	$ 1,037,736
Operating Expenses
Fuel	115,534	91,754	246,427	218,226
Purchased Power	49,564	38,903	102,317	107,039
Transmission and Other PPFAC Recoverable Costs	14,367	10,285	34,067	27,167
Increase (Decrease) to Reflect PPFAC Recovery Treatment	3,681	(9,166)	9,087	(24,773)
Total Fuel and Purchased Power	183,146	131,776	391,898	327,659
Operations and Maintenance	89,319	89,862	265,920	256,493
Depreciation	39,929	38,302	118,223	114,667
Amortization	6,898	5,463	18,940	16,323
Taxes Other Than Income Taxes	14,321	13,549	42,800	40,329
Total Operating Expenses	333,613	278,952	837,781	755,471
Operating Income	126,147	138,258	251,315	282,265
Other Income (Expense)
Interest Expense	(16,626)	(16,291)	(49,818)	(48,972)
Allowance For Borrowed Funds	757	525	2,150	1,645
Allowance For Equity Funds	2,113	1,353	5,290	4,145
Other, Net	1,702	255	2,957	8,562
Total Other Income (Expense)	(12,054)	(14,158)	(39,421)	(34,620)
Income Before Income Tax Expense	114,093	124,100	211,894	247,645
Income Tax Expense	19,120	42,100	35,521	83,951
Net Income	$ 94,973	$ 82,000	$ 176,373	$ 163,694