STATEMENTS OF INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Income Statement [Abstract]
Operating revenues	$ 448,400	$ 374,257
Operating expenses
Electric fuel and purchased power	289,204	231,234
Operating and maintenance expenses	69,398	64,570
Demand side management program expenses	3,064	3,040
Depreciation and amortization	30,512	30,205
Taxes (other than income taxes)	13,646	12,149
Total operating expenses	405,824	341,198
Operating income	42,576	33,059
Other income (expense), net	41	(48)
Allowance for funds used during construction — equity	3,640	2,622
Interest charges and financing costs
Interest charges — includes other financing costs of $730 and $736, respectively	19,281	17,773
Allowance for funds used during construction — debt	(2,127)	(1,609)
Total interest charges and financing costs	17,154	16,164
Income before income taxes	29,103	19,469
Income taxes	10,368	6,885
Net income	$ 18,735	$ 12,584