STATEMENTS OF INCOME (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
STATEMENTS OF INCOME [Abstract]
Operating revenues	$ 1,540,055	$ 1,707,565	$ 1,612,990
Operating expenses
Electric fuel and purchased power	889,567	1,089,415	1,023,938
Operating and maintenance expenses	251,853	251,886	249,071
Demand side management program expenses	12,891	15,415	11,625
Depreciation and amortization	113,743	106,974	103,935
Taxes (other than income taxes)	46,246	43,278	40,984
Total operating expenses	1,314,300	1,506,968	1,429,553
Operating income	225,755	200,597	183,437
Other income, net	46	406	27
Allowance for funds used during construction - equity	7,272	5,342	4,188
Interest charges and financing costs
Interest charges - includes other financing costs of $2,996, $2,964 and $2,635, respectively	69,074	65,095	63,912
Allowance for funds used during construction - debt	(4,599)	(3,784)	(3,193)
Total interest charges and financing costs	64,475	61,311	60,719
Income before income taxes	168,598	145,034	126,933
Income taxes	62,229	55,133	48,866
Net income	$ 106,369	$ 89,901	$ 78,067