STATEMENTS OF INCOME (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Operating revenues	$ 1,937,370	$ 1,707,087	$ 1,540,055
Operating expenses
Electric fuel and purchased power	1,192,176	1,059,330	889,567
Operating and maintenance expenses	277,217	253,880	251,853
Demand side management program expenses	12,350	12,420	12,891
Depreciation and amortization	135,632	121,907	113,743
Taxes (other than income taxes)	53,871	49,533	46,246
Total operating expenses	1,671,246	1,497,070	1,314,300
Operating income	266,124	210,017	225,755
Other (expense) income, net	(59)	140	46
Allowance for funds used during construction — equity	12,118	10,186	7,272
Interest charges and financing costs
Interest charges — includes other financing costs of $3,038, $3,031 and $2,996, respectively	80,218	77,866	69,074
Allowance for funds used during construction — debt	(7,089)	(6,461)	(4,599)
Total interest charges and financing costs	73,129	71,405	64,475
Income before income taxes	205,054	148,938	168,598
Income taxes	75,202	53,761	62,229
Net income	$ 129,852	$ 95,177	$ 106,369