Quarterly Summary of Operations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	0 Months Ended	3 Months Ended								12 Months Ended
	Dec. 20, 2013	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating Revenues:
Regulated electric revenues		$ 93,781	$ 119,836	$ 96,846	$ 93,494	$ 87,176	$ 118,914	$ 96,339	$ 89,936	$ 403,957	$ 392,365	$ 375,858
Regulated gas revenues		60,089	18,864	30,042	72,467	47,492	16,587	18,629	57,019	181,462	139,727	165,271
Nonregulated revenues		1,393	1,399	1,400	1,276	2,337	2,337	2,253	2,304	5,468	9,231	5,253
Total Operating Revenues		155,263	140,099	128,288	167,237	137,005	137,838	117,221	149,259	590,887	541,323	546,382
Operating expenses		129,301	100,199	103,791	129,609	117,655	97,950	92,140	120,815	462,900	428,560	438,514
Operating Income		25,962	39,900	24,497	37,628	19,350	39,888	25,081	28,444	127,987	112,763	107,868
Interest and other income, net		(4,056)	(855)	(1,946)	(1,366)	(2,656)	(1,993)	(2,215)	(2,534)
Income tax provision		(7,829)	(14,692)	(8,660)	(13,678)	(6,208)	(14,253)	(8,596)	(9,862)	(44,859)	(38,919)	(35,992)
Net Income		$ 14,077	$ 24,353	$ 13,891	$ 22,584	$ 10,486	$ 23,642	$ 14,270	$ 16,048	$ 74,905	$ 64,446	$ 60,928
Earnings per common share		$ 0.41	$ 0.70	$ 0.40	$ 0.65	$ 0.30	$ 0.68	$ 0.41	$ 0.47	$ 2.16	$ 1.86	$ 1.76
Dividends per share		$ 0.272	$ 0.272	$ 0.263	$ 0.263	$ 0.263	$ 0.263	$ 0.255	$ 0.255	$ 1.07	$ 1.04	$ 1.01
Stock split declared December 20, 2013, ratio (3:2)	1.5