STATEMENTS OF INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Statement [Abstract]
Operating revenues	$ 481,407	$ 471,889	$ 1,317,495	$ 1,192,578
Operating expenses
Electric fuel and purchased power	293,831	259,783	814,077	676,079
Operating and maintenance expenses	66,288	60,676	198,309	185,961
Demand side management program expenses	2,966	3,129	9,216	9,366
Depreciation and amortization	30,315	28,690	91,575	84,785
Taxes (other than income taxes)	13,354	12,102	38,137	35,142
Total operating expenses	406,754	364,380	1,151,314	991,333
Operating income	74,653	107,509	166,181	201,245
Other expense, net	(115)	(73)	(58)	(53)
Allowance for funds used during construction — equity	2,150	1,670	6,972	5,062
Interest charges and financing costs
Interest charges — includes other financing costs of $773, $721, $2,248 and $2,243, respectively	22,892	17,649	58,509	51,290
Allowance for funds used during construction — debt	(1,387)	(1,052)	(4,406)	(3,208)
Total interest charges and financing costs	21,505	16,597	54,103	48,082
Income before income taxes	55,183	92,509	118,992	158,172
Income taxes	20,146	34,262	43,165	58,271
Net income	$ 35,037	$ 58,247	$ 75,827	$ 99,901