STATEMENTS OF INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Operating revenues	$ 1,918,000	$ 1,850,959	$ 1,787,218
Operating expenses
Electric fuel and purchased power	1,055,333	1,034,950	1,001,083
Operating and maintenance expenses	289,555	269,471	289,856
Demand side management program expenses	15,525	16,028	13,365
Depreciation and amortization	193,915	162,429	150,913
Taxes (other than income taxes)	66,863	60,800	57,536
Total operating expenses	1,621,191	1,543,678	1,512,753
Operating income	296,809	307,281	274,465
Other income (expense), net	2,359	91	(6)
Allowance for funds used during construction — equity	9,310	9,981	7,378
Interest charges and financing costs
Interest charges — includes other financing costs of $2,491, $3,055 and $3,158, respectively	86,233	88,671	84,040
Public Utilities, Allowance For Funds Used During Construction, Capitalized Cost Of Debt	(5,384)	(5,589)	(4,491)
Total interest charges and financing costs	80,849	83,082	79,549
Income before income taxes	227,629	234,271	202,288
Income taxes	68,416	82,114	75,025
Net income	$ 159,213	$ 152,157	$ 127,263