STATEMENTS OF INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
STATEMENTS OF INCOME (UNAUDITED) [Abstract]
Operating revenues	$ 471,889	$ 522,921	$ 1,192,578	$ 1,337,419
Operating expenses
Electric fuel and purchased power	259,783	325,930	676,079	850,583
Operating and maintenance expenses	60,676	61,651	185,961	187,136
Demand side management program expenses	3,129	4,658	9,366	12,469
Depreciation and amortization	28,690	27,067	84,785	79,589
Taxes (other than income taxes)	12,102	10,730	35,142	31,825
Total operating expenses	364,380	430,036	991,333	1,161,602
Operating income	107,509	92,885	201,245	175,817
Other (expense) income, net	(73)	286	(53)	432
Allowance for funds used during construction - equity	1,670	894	5,062	4,178
Interest charges and financing costs
Interest charges - includes other financing costs of $721, $763, $2,243 and $2,188, respectively	17,649	16,456	51,290	48,322
Allowance for funds used during construction - debt	(1,052)	(655)	(3,208)	(3,022)
Total interest charges and financing costs	16,597	15,801	48,082	45,300
Income before income taxes	92,509	78,264	158,172	135,127
Income taxes	34,262	29,683	58,271	51,636
Net income	$ 58,247	$ 48,581	$ 99,901	$ 83,491