CONSOLIDATED STATEMENTS OF INCOME (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating revenues
Electric	$ 829,748	$ 789,168	$ 757,565
Natural gas	169,629	132,867	103,100
Other	1,085	1,003	1,177
Total operating revenues	1,000,462	923,038	861,842
Operating expenses
Electric fuel and purchased power, non-affiliates	19,595	18,129	19,440
Purchased power, affiliates	425,471	416,173	405,016
Cost of natural gas sold and transported	114,250	81,572	61,370
Operating and maintenance expenses	191,213	175,522	167,503
Conservation program expenses	11,537	12,333	14,442
Depreciation and amortization	79,654	76,897	69,234
Taxes (other than income taxes)	27,114	25,231	24,971
Total operating expenses	868,834	805,857	761,976
Operating income	131,628	117,181	99,866
Other income, net	270	253	476
Allowance for funds used during construction — equity	7,060	4,259	2,104
Interest charges and financing costs
Interest charges — includes other financing costs of $1,570, $1,538, and $1,574, respectively	29,273	27,797	24,799
Allowance for funds used during construction — debt	(3,360)	(1,981)	(1,862)
Total interest charges and financing costs	25,913	25,816	22,937
Income before income taxes	113,045	95,877	79,509
Income taxes	42,403	36,409	29,558
Net income	$ 70,642	$ 59,468	$ 49,951