STATEMENTS OF INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Operating Revenues	$ 461,055	$ 534,290	$ 501,875
Operating Expenses:
Cost of Sales (Excluding depreciation)	174,390	245,290	231,216
Operations	95,609	107,836	102,428
Maintenance	17,549	16,183	13,457
Depreciation	47,432	41,365	37,324
Energy and Other Taxes	3,620	4,031	3,760
Total Operating Expenses	338,600	414,705	388,185
Operating Income	122,455	119,585	113,690
Other Income and Expense	3,831	3,844	5,560
Interest Charges	(17,875)	(19,906)	(17,872)
Income Before Income Taxes	108,411	103,523	101,378
Income Taxes	(39,366)	(36,945)	(34,895)
Net Income	$ 69,045	$ 66,578	$ 66,483