STATEMENTS OF INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Operating revenues	$ 1,787,218	$ 1,937,370	$ 1,707,087
Operating expenses
Electric fuel and purchased power	1,001,083	1,192,176	1,059,330
Operating and maintenance expenses	289,856	277,217	253,880
Demand side management program expenses	13,365	12,350	12,420
Depreciation and amortization	150,913	135,632	121,907
Taxes (other than income taxes)	57,536	53,871	49,533
Total operating expenses	1,512,753	1,671,246	1,497,070
Operating income	274,465	266,124	210,017
Other (expense) income, net	(6)	(59)	140
Allowance for funds used during construction — equity	7,378	12,118	10,186
Interest charges and financing costs
Interest charges — includes other financing costs of $3,158, $3,038 and $3,031, respectively	84,040	80,218	77,866
Public Utilities, Allowance For Funds Used During Construction, Capitalized Cost Of Debt	4,491	7,089	6,461
Total interest charges and financing costs	79,549	73,129	71,405
Income before income taxes	202,288	205,054	148,938
Income taxes	75,025	75,202	53,761
Net income	$ 127,263	$ 129,852	$ 95,177