STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Operating revenues	$ 460,072	$ 390,839
Operating expenses
Electric fuel and purchased power	253,685	213,403
Operating and maintenance expenses	76,889	64,123
Demand side management program expenses	3,875	3,377
Depreciation and amortization	50,418	40,331
Taxes (other than income taxes)	16,790	16,036
Total operating expenses	401,657	337,270
Operating income	58,415	53,569
Other income, net	31	34
Allowance for funds used during construction — equity	2,135	2,337
Interest charges and financing costs
Interest charges — includes other financing costs of $581 and $820, respectively	22,738	21,932
Allowance for funds used during construction — debt	(1,339)	(1,327)
Total interest charges and financing costs	21,399	20,605
Income before income taxes	39,182	35,335
Income taxes	14,127	12,812
Net income	$ 25,055	$ 22,523