Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Data [Abstract]
Operating revenues	$ 11,617	$ 12,368	$ 11,895	$ 11,209	$ 11,499		$ 12,062	$ 11,768	$ 10,571	$ 47,089	$ 45,900	$ 42,383
Operating income	5,492	6,353	5,682	5,134	5,427		6,092	5,800	4,758	22,661	22,077	20,761
Net income	$ 3,516	$ 3,520	$ 2,925	$ 2,528	$ 3,551	[1]	$ 3,065	$ 2,757	$ 2,111	$ 12,489	$ 11,484	$ 9,654
Basic earnings per share (in dollars per share)	$ 0.27	$ 0.28	$ 0.22	$ 0.20	$ 0.28		$ 0.23	$ 0.22	$ 0.16	$ 0.97	$ 0.89	$ 0.75
Dividends declared per share (in dollars per share)	$ 0.1555	$ 0.1495	$ 0.1495	$ 0.1495	$ 0.1495		$ 0.1431	$ 0.1431	$ 0.1431	$ 0.6040	$ 0.5788	$ 0.5580
IRS TPR ongoing deduction										$ (1,438)	$ (1,342)	$ 0

[1] Fourth quarter net income includes a tax benefit of $1,342 for the full year 2014 due to the implementation of the IRS TPR.