STATEMENTS OF INCOME (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
STATEMENTS OF INCOME [Abstract]
Operating revenues	$ 1,707,565	$ 1,612,990	$ 1,459,223
Operating expenses
Electric fuel and purchased power	1,089,415	1,023,938	914,350
Operating and maintenance expenses	251,886	249,071	221,681
Demand side management program expenses	15,415	11,625	7,270
Depreciation and amortization	106,974	103,935	104,602
Taxes (other than income taxes)	43,278	40,984	38,503
Total operating expenses	1,506,968	1,429,553	1,286,406
Operating income	200,597	183,437	172,817
Other income, net	406	27	264
Allowance for funds used during construction - equity	5,342	4,188	4,082
Interest charges and financing costs
Interest charges - includes other financing costs of $2,964, $2,635 and $2,653, respectively	65,095	63,912	71,688
Allowance for funds used during construction - debt	(3,784)	(3,193)	(2,770)
Total interest charges and financing costs	61,311	60,719	68,918
Income before income taxes	145,034	126,933	108,245
Income taxes	55,133	48,866	40,495
Net income	$ 89,901	$ 78,067	$ 67,750