STATEMENTS OF INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Statement [Abstract]
Operating revenues	$ 492,536	$ 461,831	$ 940,936	$ 836,088
Operating expenses
Electric fuel and purchased power	314,146	289,012	603,350	520,246
Operating and maintenance expenses	68,963	67,451	138,361	132,021
Demand side management program expenses	2,849	3,210	5,913	6,250
Depreciation and amortization	35,071	31,055	65,583	61,260
Taxes (other than income taxes)	12,507	12,634	26,153	24,783
Total operating expenses	433,536	403,362	839,360	744,560
Operating income	59,000	58,469	101,576	91,528
Other (expense) income, net	(129)	105	(88)	57
Allowance for funds used during construction — equity	2,895	2,200	6,535	4,822
Interest charges and financing costs
Interest charges — includes other financing costs of $731, $739, $1,461 and $1,475, respectively	19,645	17,844	38,926	35,617
Allowance for funds used during construction — debt	(1,721)	(1,410)	(3,848)	(3,019)
Total interest charges and financing costs	17,924	16,434	35,078	32,598
Income before income taxes	43,842	44,340	72,945	63,809
Income taxes	15,807	16,134	26,175	23,019
Net income	$ 28,035	$ 28,206	$ 46,770	$ 40,790