STATEMENTS OF INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Operating revenues	$ 1,850,959	$ 1,787,218	$ 1,937,370
Operating expenses
Electric fuel and purchased power	1,034,950	1,001,083	1,192,176
Operating and maintenance expenses	269,471	289,856	277,217
Demand side management program expenses	16,028	13,365	12,350
Depreciation and amortization	162,429	150,913	135,632
Taxes (other than income taxes)	60,800	57,536	53,871
Total operating expenses	1,543,678	1,512,753	1,671,246
Operating income	307,281	274,465	266,124
Other income (expense), net	91	(6)	(59)
Allowance for funds used during construction — equity	9,981	7,378	12,118
Interest charges and financing costs
Interest charges — includes other financing costs of $3,055, $3,158 and $3,038, respectively	88,671	84,040	80,218
Public Utilities, Allowance For Funds Used During Construction, Capitalized Cost Of Debt	(5,589)	(4,491)	(7,089)
Total interest charges and financing costs	83,082	79,549	73,129
Income before income taxes	234,271	202,288	205,054
Income taxes	82,114	75,025	75,202
Net income	$ 152,157	$ 127,263	$ 129,852