Quarterly Summary of Operations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating Revenues:
Regulated electric revenues	$ 84,054	$ 114,963	$ 90,834	$ 86,007	$ 81,448	$ 109,849	$ 87,438	$ 81,994	$ 375,858	$ 360,729	$ 332,324
Regulated gas revenues	45,523	17,249	25,062	77,437	51,684	17,394	20,290	76,547	165,271	165,915	192,334
Nonregulated revenues	1,369	1,360	1,363	1,161	2,796	695	1,354	1,102	5,253	5,947	9,161
Total Operating Revenues	130,946	133,572	117,259	164,605	135,928	127,938	109,082	159,643	546,382	532,591	533,819
Operating expenses	113,507	97,238	94,062	133,707	115,575	93,586	88,960	137,868	438,514	435,989	449,154
Operating Income	17,439	36,334	23,197	30,898	20,353	34,352	20,122	21,775	107,868	96,602	84,665
Interest and other income, net	(2,781)	(2,801)	(2,902)	(2,464)	(2,326)	(1,453)	(2,098)	813
Income tax provision	(5,274)	(12,495)	(7,572)	(10,651)	(6,030)	(12,990)	(6,472)	(8,328)	(35,992)	(33,820)	(28,170)
Net Income	$ 9,384	$ 21,038	$ 12,723	$ 17,783	$ 11,997	$ 19,909	$ 11,552	$ 14,260	$ 60,928	$ 57,718	$ 50,997
Earnings per common share	$ 0.41	$ 0.91	$ 0.55	$ 0.77	$ 0.52	$ 0.86	$ 0.50	$ 0.62	$ 2.64	$ 2.50	$ 2.21
Dividends per share	$ 0.383	$ 0.383	$ 0.375	$ 0.375	$ 0.375	$ 0.375	$ 0.368	$ 0.368	$ 1.52	$ 1.49	$ 1.46