STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Operating revenues	$ 554,926	$ 530,752	$ 1,386,210	$ 1,377,566
Operating expenses
Electric fuel and purchased power	297,587	287,476	757,537	776,301
Operating and maintenance expenses	71,699	72,036	202,410	219,760
Demand side management program expenses	5,663	3,726	12,279	10,155
Depreciation and amortization	42,026	35,422	123,250	107,911
Taxes (other than income taxes)	15,589	15,016	46,417	43,472
Total operating expenses	432,564	413,676	1,141,893	1,157,599
Operating income	122,362	117,076	244,317	219,967
Other income, net	137	103	563	203
Allowance for funds used during construction — equity	2,632	2,085	7,348	5,578
Interest charges and financing costs
Interest charges — includes other financing costs of $828, $789, $2,461 and $2,334, respectively	23,343	21,779	67,350	63,737
Allowance for funds used during construction — debt	(1,422)	(1,204)	(4,146)	(3,402)
Total interest charges and financing costs	21,921	20,575	63,204	60,335
Income before income taxes	103,210	98,689	189,024	165,413
Income taxes	34,864	36,874	65,944	60,775
Net income	$ 68,346	$ 61,815	$ 123,080	$ 104,638