STATEMENTS OF INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Income Statement [Abstract]
Operating revenues	$ 461,831	$ 380,201	$ 836,088	$ 720,689
Operating expenses
Electric fuel and purchased power	289,012	213,035	520,246	416,296
Operating and maintenance expenses	67,451	62,145	132,021	125,285
Demand side management program expenses	3,210	3,160	6,250	6,237
Depreciation and amortization	31,055	28,249	61,260	56,095
Taxes (other than income taxes)	12,634	11,721	24,783	23,040
Total operating expenses	403,362	318,310	744,560	626,953
Operating income	58,469	61,891	91,528	93,736
Other income, net	105	142	57	20
Allowance for funds used during construction — equity	2,200	1,702	4,822	3,392
Interest charges and financing costs
Interest charges – includes other financing costs of $739, $754, $1,475 and $1,522, respectively	17,844	16,935	35,617	33,641
Allowance for funds used during construction — debt	(1,410)	(1,073)	(3,019)	(2,156)
Total interest charges and financing costs	16,434	15,862	32,598	31,485
Income before income taxes	44,340	47,873	63,809	65,663
Income taxes	16,134	17,579	23,019	24,009
Net income	$ 28,206	$ 30,294	$ 40,790	$ 41,654