STATEMENTS OF INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
STATEMENTS OF INCOME (UNAUDITED) [Abstract]
Operating revenues	$ 374,257	$ 340,488
Operating expenses
Electric fuel and purchased power	231,234	203,261
Operating and maintenance expenses	64,570	63,140
Demand side management program expenses	3,040	3,077
Depreciation and amortization	30,205	27,846
Taxes (other than income taxes)	12,149	11,319
Total operating expenses	341,198	308,643
Operating income	33,059	31,845
Other expense, net	(48)	(122)
Allowance for funds used during construction - equity	2,622	1,690
Interest charges and financing costs
Interest charges - includes other financing costs of $736 and $768, respectively	17,773	16,706
Allowance for funds used during construction - debt	(1,609)	(1,083)
Total interest charges and financing costs	16,164	15,623
Income before income taxes	19,469	17,790
Income taxes	6,885	6,430
Net income	$ 12,584	$ 11,360