Quarterly Summary of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Revenues:
Electric revenues	$ 90,815	$ 116,568	$ 93,961	$ 93,028	$ 93,613	$ 120,821	$ 97,077	$ 97,469	$ 394,372	$ 408,980	$ 402,001
Gas revenues	45,694	18,643	23,079	56,845	47,328	17,377	25,070	70,100	144,261	159,875	157,767
Total Operating Revenues	136,509	135,211	117,040	149,873	140,941	138,198	122,147	167,569	538,633	568,855	559,768
Operating expenses	117,797	96,886	95,520	118,433	119,064	99,460	102,364	137,057	428,636	457,945	445,561
Operating Income	18,712	38,325	21,520	31,440	21,877	38,738	19,783	30,512	109,997	110,910	114,207
Interest and other income, net	454	769	1,011	(390)	(2,099)	(627)	(730)	(796)
Income tax provision	(3,370)	(7,300)	(3,740)	(5,013)	(3,116)	(7,454)	(3,505)	(5,709)	(19,423)	(19,784)	(27,434)
Net Income	$ 15,796	$ 31,794	$ 18,791	$ 26,037	$ 16,662	$ 30,657	$ 15,548	$ 24,007	$ 92,418	$ 86,874	$ 84,219
Earnings per common share	$ 0.44	$ 0.88	$ 0.53	$ 0.75	$ 0.48	$ 0.88	$ 0.45	$ 0.69	$ 2.60	$ 2.51	$ 2.43
Dividends per share	$ 0.370	$ 0.370	$ 0.353	$ 0.353	$ 0.353	$ 0.353	$ 0.338	$ 0.338	$ 1.45	$ 1.38	$ 1.32