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
Revenues [Abstract]
Operating revenues	$ 540,063	$ 551,623	$ 1,468,633	$ 1,491,491
Operating expenses
Electric fuel and purchased power	284,006	294,400	795,592	816,027
Operating and maintenance expenses	71,444	65,540	203,660	211,101
Demand side management expenses	4,590	4,236	13,527	11,802
Depreciation and amortization	52,204	47,548	150,199	144,781
Taxes (other than income taxes)	16,814	16,743	50,033	50,222
Total operating expenses	429,058	428,467	1,213,011	1,233,933
Operating income	111,005	123,156	255,622	257,558
Other expense, net	(1,026)	(464)	(2,512)	(1,795)
Allowance for funds used during construction — equity	5,019	2,453	11,637	6,457
Interest charges and financing costs
Interest charges — includes other financing costs of $710, $625, $2,106, and $1,781, respectively	21,006	21,444	61,782	66,128
Allowance for funds used during construction — debt	(2,223)	(1,349)	(5,526)	(3,816)
Total interest charges and financing costs	18,783	20,095	56,256	62,312
Income before income taxes	96,215	105,050	208,491	199,908
Income taxes	14,674	37,269	35,400	71,710
Net income	$ 81,541	$ 67,781	$ 173,091	$ 128,198