Consolidated Statements of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating revenues:
Electric	$ 592,491	$ 536,413	$ 510,653
Gas	51,842	50,041	39,849
Other	7,997	7,876	6,595
Total operating revenues	652,330	594,330	557,097
Operating revenue deductions:
Fuel and purchased power	215,086	175,406	178,896
Cost of natural gas sold and transported	27,025	25,795	18,633
Regulated operating expenses	110,691	105,333	94,371
Other operating expenses	2,987	3,142	2,730
Maintenance and repairs	46,775	40,873	40,444
Loss on plant disallowance	86	2,409
Depreciation and amortization	73,185	69,306	60,447
Provision for income taxes	39,398	37,465	34,096
Other taxes	37,098	34,938	31,259
Total operating revenue deductions	552,331	494,667	460,876
Operating income	99,999	99,663	96,221
Other income and (deductions):
Allowance for equity funds used during construction	(6,420)	(3,853)	(1,147)
Interest income	51	566	972
Provision for other income taxes	178	(27)	(63)
Other - non-operating expense, net	(1,302)	(1,218)	(1,910)
Total other income and (deductions)	5,347	3,174	146
Interest charges:
Long-term debt	40,637	40,354	40,192
Short-term debt	113	60	187
Allowance for borrowed funds used during construction	(3,497)	(2,087)	(781)
Other	990	1,065	1,088
Total interest charges	38,243	39,392	40,686
Net income	$ 67,103	$ 63,445	$ 55,681
Weighted average number of common shares outstanding - basic (in shares)	43,291	42,781	42,257
Weighted average number of common shares outstanding - diluted (in shares)	43,314	42,803	42,284
Total earnings per weighted average share of common stock - basic and diluted (in dollars per share)	$ 1.55	$ 1.48	$ 1.32
Dividends paid per share common stock (in dollars per share)	$ 1.025	$ 1.005	$ 1.000