STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Operating revenues	$ 422,985	$ 492,536	$ 846,814	$ 940,936
Operating expenses
Electric fuel and purchased power	243,026	314,146	488,825	603,350
Operating and maintenance expenses	73,827	68,963	147,724	138,361
Demand side management program expenses	2,760	2,849	6,429	5,913
Depreciation and amortization	36,750	35,071	72,489	65,583
Taxes (other than income taxes)	13,490	12,507	28,456	26,153
Total operating expenses	369,853	433,536	743,923	839,360
Operating income	53,132	59,000	102,891	101,576
Other income (expense), net	156	(129)	100	(88)
Allowance for funds used during construction — equity	1,788	2,895	3,493	6,535
Interest charges and financing costs
Interest charges — includes other financing costs of $771, $731, $1,544 and $1,461, respectively	21,074	19,645	41,958	38,926
Allowance for funds used during construction — debt	(1,137)	(1,721)	(2,198)	(3,848)
Total interest charges and financing costs	19,937	17,924	39,760	35,078
Income before income taxes	35,139	43,842	66,724	72,945
Income taxes	12,563	15,807	23,901	26,175
Net income	$ 22,576	$ 28,035	$ 42,823	$ 46,770