STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Operating revenues	$ 530,752	$ 552,779	$ 1,377,566	$ 1,493,715
Operating expenses
Electric fuel and purchased power	287,476	315,524	776,301	918,874
Operating and maintenance expenses	72,036	66,833	219,760	205,194
Demand side management program expenses	3,726	3,455	10,155	9,368
Depreciation and amortization	35,422	34,207	107,911	99,790
Taxes (other than income taxes)	15,016	13,991	43,472	40,144
Total operating expenses	413,676	434,010	1,157,599	1,273,370
Operating income	117,076	118,769	219,967	220,345
Other income (expense), net	103	66	203	(22)
Allowance for funds used during construction — equity	2,085	3,147	5,578	9,682
Interest charges and financing costs
Interest charges — includes other financing costs of $789, $768, $2,334 and $2,229, respectively	21,779	20,479	63,737	59,405
Public Utilities, Allowance For Funds Used During Construction, Capitalized Cost Of Debt	(1,204)	(1,846)	(3,402)	(5,694)
Total interest charges and financing costs	20,575	18,633	60,335	53,711
Income before income taxes	98,689	103,349	165,413	176,294
Income taxes	36,874	36,412	60,775	62,587
Net income	$ 61,815	$ 66,937	$ 104,638	$ 113,707