STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Operating revenues	$ 551,623	$ 554,926	$ 1,491,491	$ 1,386,210
Operating expenses
Electric fuel and purchased power	294,400	297,587	816,027	757,537
Operating and maintenance expenses	66,289	71,699	213,348	202,410
Demand side management expenses	4,236	5,663	11,802	12,279
Depreciation and amortization	47,548	42,026	144,781	123,250
Taxes (other than income taxes)	16,743	15,589	50,222	46,417
Total operating expenses	429,216	432,564	1,236,180	1,141,893
Operating income	122,407	122,362	255,311	244,317
Other income, net	285	137	452	563
Allowance for funds used during construction — equity	2,453	2,632	6,457	7,348
Interest charges and financing costs
Interest charges — includes other financing costs of $625, $828, $1,781, and $2,461, respectively	21,444	23,343	66,128	67,350
Allowance for funds used during construction — debt	(1,349)	(1,422)	(3,816)	(4,146)
Total interest charges and financing costs	20,095	21,921	62,312	63,204
Income before income taxes	105,050	103,210	199,908	189,024
Income taxes	37,269	34,864	71,710	65,944
Net income	$ 67,781	$ 68,346	$ 128,198	$ 123,080