STATEMENTS OF INCOME (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Statement [Abstract]
Operating revenues	$ 1,707,087	$ 1,540,055	$ 1,707,565
Operating expenses
Electric fuel and purchased power	1,059,330	889,567	1,089,415
Operating and maintenance expenses	253,880	251,853	251,886
Demand side management program expenses	12,420	12,891	15,415
Depreciation and amortization	121,907	113,743	106,974
Taxes (other than income taxes)	49,533	46,246	43,278
Total operating expenses	1,497,070	1,314,300	1,506,968
Operating income	210,017	225,755	200,597
Other income, net	140	46	406
Allowance for funds used during construction — equity	10,186	7,272	5,342
Interest charges and financing costs
Interest charges — includes other financing costs of $3,031, $2,996 and $2,964, respectively	77,866	69,074	65,095
Allowance for funds used during construction — debt	(6,461)	(4,599)	(3,784)
Total interest charges and financing costs	71,405	64,475	61,311
Income before income taxes	148,938	168,598	145,034
Income taxes	53,761	62,229	55,133
Net income	$ 95,177	$ 106,369	$ 89,901