STATEMENTS OF INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Statement [Abstract]
Operating revenues	$ 423,829	$ 448,400
Operating expenses
Electric fuel and purchased power	245,799	289,204
Operating and maintenance expenses	73,897	69,398
Demand side management program expenses	3,669	3,064
Depreciation and amortization	35,739	30,512
Taxes (other than income taxes)	14,966	13,646
Total operating expenses	374,070	405,824
Operating income	49,759	42,576
Other (expense) income, net	(56)	41
Allowance for funds used during construction — equity	1,705	3,640
Interest charges and financing costs
Interest charges — includes other financing costs of $774 and $730, respectively	20,884	19,281
Allowance for funds used during construction — debt	(1,061)	(2,127)
Total interest charges and financing costs	19,823	17,154
Income before income taxes	31,585	29,103
Income taxes	11,338	10,368
Net income	$ 20,247	$ 18,735