Selected Quarterly Financial Data (Unaudited) (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
Quarterly Financial Data [Abstract]
Operating revenues	$ 12,353	$ 12,692	$ 12,254	$ 11,290	$ 11,885	$ 12,601	$ 11,820	$ 11,278	$ 48,589	$ 47,584	$ 47,089
Operating income	5,940	6,147	5,529	4,857	5,565	6,414	5,695	5,214	22,473	22,888	22,661
Net income	$ 3,527	$ 3,931	$ 2,935	$ 2,581	$ 2,942	$ 3,571	$ 2,847	$ 2,486	$ 12,974	$ 11,846	$ 12,489
Basic earnings per share (in dollars per share)	$ 0.27	$ 0.31	$ 0.23	$ 0.20	$ 0.23	$ 0.27	$ 0.23	$ 0.19	$ 1.01	$ 0.92	$ 0.97
Diluted earnings per share (in dollars per share)	0.27	0.31	0.23	0.20	0.23	0.27	0.23	0.19	1.01	0.92	0.97
Dividends declared per share (in dollars per share)	$ 0.1666	$ 0.1602	$ 0.1602	$ 0.1602	$ 0.1602	$ 0.1555	$ 0.1555	$ 0.1555	$ 0.6472	$ 0.6267	$ 0.6040