Quarterly Summary of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Revenues:
Electric revenues	$ 93,613	$ 120,821	$ 97,077	$ 97,469	$ 88,464	$ 119,388	$ 99,282	$ 94,867	$ 408,980	$ 402,001	$ 414,274
Gas revenues	47,328	17,377	25,070	70,100	51,615	18,407	24,980	62,765	159,875	157,767	148,825
Total Operating Revenues	140,941	138,198	122,147	167,569	140,079	137,795	124,262	157,632	568,855	559,768	563,099
Operating expenses	119,064	99,460	102,364	137,057	116,089	97,997	100,031	131,444	457,945	445,561	438,474
Operating Income (Loss)	21,877	38,738	19,783	30,512	23,990	39,798	24,231	26,188	110,910	114,207	124,625
Interest and other income, net	(2,099)	(627)	(730)	(796)	(1,987)	(695)	(52)	180
Income tax provision	(3,116)	(7,454)	(3,505)	(5,709)	(5,642)	(9,597)	(5,828)	(6,367)	(19,784)	(27,434)	(22,094)	[1]
Net Income	$ 16,662	$ 30,657	$ 15,548	$ 24,007	$ 16,361	$ 29,506	$ 18,351	$ 20,001	$ 86,874	$ 84,219	$ 97,606
Earnings per common share	$ 0.48	$ 0.88	$ 0.45	$ 0.69	$ 0.47	$ 0.85	$ 0.53	$ 0.58	$ 2.51	$ 2.43	$ 2.82
Dividends per share	$ 0.353	$ 0.353	$ 0.338	$ 0.338	$ 0.338	$ 0.338	$ 0.323	$ 0.323	$ 1.38	$ 1.32	$ 1.26
Effects Of The Tax Cuts And Jobs Act
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act											$ 21,700

[1] In December 2017, there was a $21.7 million one-time tax impact as a result of the Tax Act.