STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Operating revenues	$ 447,232	$ 460,072
Operating expenses
Electric fuel and purchased power	253,944	253,685
Operating and maintenance expenses	66,068	76,140
Demand side management expenses	4,158	3,875
Depreciation and amortization	48,416	50,418
Taxes (other than income taxes)	17,590	16,790
Total operating expenses	390,176	400,908
Operating income	57,056	59,164
Other expense, net	(704)	(718)
Allowance for funds used during construction — equity	3,417	2,135
Interest charges and financing costs
Interest charges — includes other financing costs of $694, and $581, respectively	20,155	22,738
Allowance for funds used during construction — debt	(1,771)	(1,339)
Total interest charges and financing costs	18,384	21,399
Income before income taxes	41,385	39,182
Income taxes	8,286	14,127
Net income	$ 33,099	$ 25,055