Quarterly Summary of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Operating Revenues:
Electric revenues	$ 92,734		$ 120,761	$ 102,382	$ 98,397	$ 96,750	$ 119,147	$ 100,615	$ 93,690	$ 414,274		$ 410,202	$ 420,291
Gas revenues	47,540		18,778	24,081	58,426	42,175	17,570	20,961	53,837	148,825		134,543	143,737
Total Operating Revenues	140,274		139,539	126,463	156,823	138,925	136,717	121,576	147,527	563,099		544,745	564,028
Operating expenses	114,000		96,538	99,912	123,913	114,967	90,190	93,364	117,996	434,363		416,517	439,773
Operating Income (Loss)	26,274		43,001	26,551	32,910	23,958	46,527	28,212	29,531	128,736		128,228	124,255
Interest and other income, net	(3,533)		(788)	(2,272)	(2,443)	(1,886)	(2,933)	(2,778)	(2,558)
Income tax provision	13,393	[1]	(15,584)	(8,736)	(11,167)	(7,570)	(15,714)	(9,284)	(9,945)	(22,094)	[1]	(42,513)	(41,363)
Net Income	$ 36,134		$ 26,629	$ 15,543	$ 19,300	$ 14,502	$ 27,880	$ 16,150	$ 17,028	$ 97,606		$ 75,560	$ 71,343
Earnings per common share	$ 1.04		$ 0.77	$ 0.45	$ 0.56	$ 0.42	$ 0.80	$ 0.47	$ 0.49	$ 2.82		$ 2.18	$ 2.06
Dividends per share	$ 0.323		$ 0.323	$ 0.308	$ 0.308	$ 0.308	$ 0.308	$ 0.295	$ 0.295	$ 1.26		$ 1.21	$ 1.16
Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act
Deferred Income Tax Expense 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. See Footnote 12 for further information.