STATEMENTS OF INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement [Abstract]
Operating revenues	$ 552,779	$ 481,407	$ 1,493,715	$ 1,317,495
Operating expenses
Electric fuel and purchased power	315,524	293,831	918,874	814,077
Operating and maintenance expenses	66,833	66,288	205,194	198,309
Demand side management program expenses	3,455	2,966	9,368	9,216
Depreciation and amortization	34,207	30,315	99,790	91,575
Taxes (other than income taxes)	13,991	13,354	40,144	38,137
Total operating expenses	434,010	406,754	1,273,370	1,151,314
Operating income	118,769	74,653	220,345	166,181
Other income (expense), net	66	(115)	(22)	(58)
Allowance for funds used during construction — equity	3,147	2,150	9,682	6,972
Interest charges and financing costs
Interest charges — includes other financing costs of $768, $773, $2,229 and $2,248, respectively	20,479	22,892	59,405	58,509
Allowance for funds used during construction — debt	(1,846)	(1,387)	(5,694)	(4,406)
Total interest charges and financing costs	18,633	21,505	53,711	54,103
Income before income taxes	103,349	55,183	176,294	118,992
Income taxes	36,412	20,146	62,587	43,165
Net income	$ 66,937	$ 35,037	$ 113,707	$ 75,827