STATEMENTS OF INCOME (UNAUDITED) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
STATEMENTS OF INCOME (UNAUDITED) [Abstract]
Operating revenues	$ 522,921	$ 467,424	$ 1,337,419	$ 1,247,355
Operating expenses
Electric fuel and purchased power	325,930	288,386	850,583	788,089
Other operating and maintenance expenses	61,651	61,807	187,136	180,794
Demand side management program expenses	4,658	3,393	12,469	8,386
Depreciation and amortization	27,067	26,083	79,589	77,391
Taxes (other than income taxes)	10,730	10,818	31,825	30,879
Total operating expenses	430,036	390,487	1,161,602	1,085,539
Operating income	92,885	76,937	175,817	161,816
Other income (expense), net	286	(82)	432	31
Allowance for funds used during construction - equity	894	1,085	4,178	2,529
Interest charges and financing costs
Interest charges - includes other financing costs of $763, $666, $2,188 and $1,975, respectively	16,456	16,217	48,322	48,099
Allowance for funds used during construction - debt	(655)	(792)	(3,022)	(2,052)
Total interest charges and financing costs	15,801	15,425	45,300	46,047
Income before income taxes	78,264	62,515	135,127	118,329
Income taxes	29,683	23,326	51,636	47,045
Net income	$ 48,581	$ 39,189	$ 83,491	$ 71,284