STATEMENTS OF INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
STATEMENTS OF INCOME (UNAUDITED) [Abstract]
Operating revenues	$ 380,201	$ 433,289	$ 720,689	$ 814,498
Operating expenses
Electric fuel and purchased power	213,035	275,403	416,296	524,653
Operating and maintenance expenses	62,145	62,732	125,285	125,485
Demand side management program expenses	3,160	4,382	6,237	7,811
Depreciation and amortization	28,249	26,364	56,095	52,522
Taxes (other than income taxes)	11,721	10,975	23,040	21,095
Total operating expenses	318,310	379,856	626,953	731,566
Operating income	61,891	53,433	93,736	82,932
Other income, net	142	84	20	146
Allowance for funds used during construction - equity	1,702	1,358	3,392	3,284
Interest charges and financing costs
Interest charges - includes other financing costs of $754, $765, $1,522 and $1,425, respectively	16,935	16,068	33,641	31,866
Allowance for funds used during construction - debt	(1,073)	(1,018)	(2,156)	(2,367)
Total interest charges and financing costs	15,862	15,050	31,485	29,499
Income before income taxes	47,873	39,825	65,663	56,863
Income taxes	17,579	15,153	24,009	21,953
Net income	$ 30,294	$ 24,672	$ 41,654	$ 34,910