Quarterly Summary of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended												12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Operating Revenues:
Electric revenues		$ 88,464	$ 119,388	$ 99,282	$ 94,867	$ 92,734		$ 120,761		$ 102,382		$ 98,397		$ 402,001	$ 414,274		$ 410,202
Gas revenues		51,615	18,407	24,980	62,765	47,540		18,778		24,081		58,426		157,767	148,825		134,543
Total Operating Revenues		140,079	137,795	124,262	157,632	140,274		139,539		126,463		156,823		559,768	563,099		544,745
Operating expenses		116,089	97,997	100,031	131,444	115,077	[1]	97,542	[1]	100,868	[1]	124,987	[1]	445,561	438,474		420,863
Operating Income (Loss)		23,990	39,798	24,231	26,188	25,197		41,997		25,595		31,836		114,207	124,625		123,882
Interest and other income, net		(1,987)	(695)	(52)	180	(2,456)	[1]	216	[1]	(1,316)	[1]	(1,369)	[1]
Income tax provision		(5,642)	(9,597)	(5,828)	(6,367)	13,393	[2]	(15,584)	[2]	(8,736)	[2]	(11,167)	[2]	(27,434)	(22,094)	[2]	(42,513)
Net Income		$ 16,361	$ 29,506	$ 18,351	$ 20,001	$ 36,134		$ 26,629		$ 15,543		$ 19,300		$ 84,219	$ 97,606		$ 75,560
Earnings per common share		$ 0.47	$ 0.85	$ 0.53	$ 0.58	$ 1.04		$ 0.77		$ 0.45		$ 0.56		$ 2.43	$ 2.82		$ 2.18
Dividends per share		$ 0.338	$ 0.338	$ 0.323	$ 0.3225	$ 0.323		$ 0.323		$ 0.308		$ 0.308		$ 1.32	$ 1.26		$ 1.21
Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act	[3]														$ 21,651

[1] Reflects retrospective application of new accounting pronouncement related to pension and other postretirement benefits. See Footnote 2 for more information.

[2] In December 2017, there was a $21.7 million one-time tax impact as a result of the Tax Act. See Footnote 12 for further information.

[3] Generated by nonregulated activities.