STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Operating revenues	$ 479,796	$ 440,445	$ 939,868	$ 831,284
Operating expenses
Electric fuel and purchased power	267,942	246,547	521,627	459,950
Operating and maintenance expenses	70,170	66,588	147,059	130,711
Demand side management expenses	3,691	3,239	7,566	6,616
Depreciation and amortization	46,815	40,893	97,233	81,224
Taxes (other than income taxes)	16,689	14,792	33,479	30,828
Total operating expenses	405,307	372,059	806,964	709,329
Operating income	74,489	68,386	132,904	121,955
Other income, net	136	392	167	426
Allowance for funds used during construction — equity	1,869	2,379	4,004	4,716
Interest charges and financing costs
Interest charges — includes other financing costs of $581, $813, $1,156, and $1,633, respectively	21,946	22,075	44,684	44,007
Allowance for funds used during construction — debt	(1,128)	(1,397)	(2,467)	(2,724)
Total interest charges and financing costs	20,818	20,678	42,217	41,283
Income before income taxes	55,676	50,479	94,858	85,814
Income taxes	20,314	18,268	34,441	31,080
Net income	$ 35,362	$ 32,211	$ 60,417	$ 54,734