STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Operating revenues	$ 481,338	$ 479,796	$ 928,570	$ 939,868
Operating expenses
Electric fuel and purchased power	257,642	267,942	511,586	521,627
Operating and maintenance expenses	66,148	69,421	132,216	145,561
Demand side management expenses	4,779	3,691	8,937	7,566
Depreciation and amortization	49,579	46,815	97,995	97,233
Taxes (other than income taxes)	15,629	16,689	33,219	33,479
Total operating expenses	393,777	404,558	783,953	805,466
Operating income	87,561	75,238	144,617	134,402
Other expense, net	(782)	(613)	(1,486)	(1,331)
Allowance for funds used during construction — equity	3,201	1,869	6,618	4,004
Interest charges and financing costs
Interest charges — includes other financing costs of $702, $581, $1,396, and $1,156, respectively	20,621	21,946	40,776	44,684
Allowance for funds used during construction — debt	(1,532)	(1,128)	(3,303)	(2,467)
Total interest charges and financing costs	19,089	20,818	37,473	42,217
Income before income taxes	70,891	55,676	112,276	94,858
Income taxes	12,440	20,314	20,726	34,441
Net income	$ 58,451	$ 35,362	$ 91,550	$ 60,417