Quarterly Summary of Operations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Revenues:
Regulated electric revenues	$ 87,176	$ 118,914	$ 96,339	$ 89,936	$ 84,054	$ 114,963	$ 90,834	$ 86,007	$ 392,365	$ 375,858	$ 360,729
Regulated gas revenues	47,492	16,587	18,629	57,019	45,523	17,249	25,062	77,437	139,727	165,271	165,915
Nonregulated revenues	2,337	2,337	2,253	2,304	1,369	1,360	1,363	1,161	9,231	5,253	5,947
Total Operating Revenues	137,005	137,838	117,221	149,259	130,946	133,572	117,259	164,605	541,323	546,382	532,591
Operating expenses	117,655	97,950	92,140	120,815	113,507	97,238	94,062	133,707	428,560	438,514	435,989
Operating Income	19,350	39,888	25,081	28,444	17,439	36,334	23,197	30,898	112,763	107,868	96,602
Interest and other income, net	(2,656)	(1,993)	(2,215)	(2,534)	(2,781)	(2,801)	(2,902)	(2,464)
Income tax provision	(6,208)	(14,253)	(8,596)	(9,862)	(5,274)	(12,495)	(7,572)	(10,651)	(38,919)	(35,992)	(33,820)
Net Income	$ 10,486	$ 23,642	$ 14,270	$ 16,048	$ 9,384	$ 21,038	$ 12,723	$ 17,783	$ 64,446	$ 60,928	$ 57,718
Earnings per common share	$ 0.46	$ 1.02	$ 0.62	$ 0.69	$ 0.41	$ 0.91	$ 0.55	$ 0.77	$ 2.79	$ 2.64	$ 2.50
Dividends per share	$ 0.395	$ 0.395	$ 0.383	$ 0.383	$ 0.383	$ 0.383	$ 0.375	$ 0.375	$ 1.56	$ 1.52	$ 1.49