STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Statement [Abstract]
Operating revenues	$ 440,445	$ 422,985	$ 831,284	$ 846,814
Operating expenses
Electric fuel and purchased power	246,547	243,026	459,950	488,825
Operating and maintenance expenses	66,588	73,827	130,711	147,724
Demand side management program expenses	3,239	2,760	6,616	6,429
Depreciation and amortization	40,893	36,750	81,224	72,489
Taxes (other than income taxes)	14,792	13,490	30,828	28,456
Total operating expenses	372,059	369,853	709,329	743,923
Operating income	68,386	53,132	121,955	102,891
Other income, net	392	156	426	100
Allowance for funds used during construction — equity	2,379	1,788	4,716	3,493
Interest charges and financing costs
Interest charges — includes other financing costs of $813, $771, $1,633 and $1,544, respectively	22,075	21,074	44,007	41,958
Public Utilities, Allowance For Funds Used During Construction, Capitalized Cost Of Debt	(1,397)	(1,137)	(2,724)	(2,198)
Total interest charges and financing costs	20,678	19,937	41,283	39,760
Income before income taxes	50,479	35,139	85,814	66,724
Income taxes	18,268	12,563	31,080	23,901
Net income	$ 32,211	$ 22,576	$ 54,734	$ 42,823