STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Income Statement [Abstract]
Operating revenues	$ 390,839	$ 423,829
Operating expenses
Electric fuel and purchased power	213,403	245,799
Operating and maintenance expenses	64,123	73,897
Demand side management program expenses	3,377	3,669
Depreciation and amortization	40,331	35,739
Taxes (other than income taxes)	16,036	14,966
Total operating expenses	337,270	374,070
Operating income	53,569	49,759
Other income (expense), net	34	(56)
Allowance for funds used during construction — equity	2,337	1,705
Interest charges and financing costs
Interest charges — includes other financing costs of $820 and $774, respectively	21,932	20,884
Public Utilities, Allowance For Funds Used During Construction, Capitalized Cost Of Debt	(1,327)	(1,061)
Total interest charges and financing costs	20,605	19,823
Income before income taxes	35,335	31,585
Income taxes	12,812	11,338
Net income	$ 22,523	$ 20,247