Quarterly Summary of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Revenues:
Electric revenues	$ 96,750	$ 119,147	$ 100,615	$ 93,690	$ 95,264	$ 123,364	$ 101,457	$ 100,206	$ 410,202	$ 420,291	$ 398,132
Gas revenues	42,175	17,570	20,961	53,837	35,709	17,431	20,669	69,928	134,543	143,737	221,720
Total Operating Revenues	138,925	136,717	121,576	147,527	130,973	140,795	122,126	170,134	544,745	564,028	619,852
Operating expenses	114,967	90,190	93,364	117,996	108,795	94,618	98,077	138,283	416,517	439,773	481,754
Operating Income (Loss)	23,958	46,527	28,212	29,531	22,178	46,177	24,049	31,851	128,228	124,255	138,098
Interest and other income, net	(1,886)	(2,933)	(2,778)	(2,558)	(3,529)	(2,472)	(2,562)	(2,986)
Income tax provision	(7,570)	(15,714)	(9,284)	(9,945)	(7,417)	(15,351)	(8,008)	(10,587)	(42,513)	(41,363)	(48,185)
Net Income	$ 14,502	$ 27,880	$ 16,150	$ 17,028	$ 11,232	$ 28,354	$ 13,479	$ 18,278	$ 75,560	$ 71,343	$ 80,319
Earnings per common share	$ 0.42	$ 0.8	$ 0.47	$ 0.49	$ 0.32	$ 0.82	$ 0.39	$ 0.53	$ 2.18	$ 2.06	$ 2.32
Dividends per share	$ 0.308	$ 0.308	$ 0.295	$ 0.295	$ 0.295	$ 0.295	$ 0.283	$ 0.283	$ 1.21	$ 1.16	$ 1.11