Quarterly Summary of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Revenues:
Regulated electric revenues	$ 93,354	$ 121,453	$ 99,481	$ 98,240	$ 86,431	$ 112,869	$ 96,697	$ 98,852	$ 412,528	$ 394,849	$ 403,957
Regulated gas revenues	35,709	17,431	20,669	69,928	58,385	21,404	31,218	110,713	143,737	221,720	181,462
Nonregulated revenues	1,910	1,911	1,976	1,966	891	862	850	680	7,763	3,283	5,468
Total Operating Revenues	130,973	140,795	122,126	170,134	145,707	135,135	128,765	210,245	564,028	619,852	590,887
Operating expenses	108,795	94,618	98,077	138,283	116,141	95,015	104,324	166,274	439,773	481,754	462,900
Operating Income (Loss)	22,178	46,177	24,049	31,851	29,566	40,120	24,441	43,971	124,255	138,098	127,987
Interest and other income, net	(3,529)	(2,472)	(2,562)	(2,986)	(5,780)	(2,505)	(1,320)	11
Income tax provision	(7,417)	(15,351)	(8,008)	(10,587)	(8,600)	(14,286)	(9,034)	(16,265)	(41,363)	(48,185)	(44,859)
Net Income	$ 11,232	$ 28,354	$ 13,479	$ 18,278	$ 15,186	$ 23,329	$ 14,087	$ 27,717	$ 71,343	$ 80,319	$ 74,905
Earnings per common share	$ 0.32	$ 0.82	$ 0.39	$ 0.53	$ 0.44	$ 0.67	$ 0.41	$ 0.8	$ 2.06	$ 2.32	$ 2.16
Dividends per share	$ 0.295	$ 0.295	$ 0.283	$ 0.283	$ 0.283	$ 0.283	$ 0.272	$ 0.272	$ 1.16	$ 1.11	$ 1.07