Quarterly Operating Results - Unaudited (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Operating Revenues	$ 31,808	$ 37,754	$ 32,725	$ 30,579	$ 30,925	$ 34,654	$ 31,666	$ 28,780	$ 132,906	$ 126,025	$ 117,139
Operating Income	7,846	14,156	10,328	8,302	8,460	11,460	9,235	6,685	40,632	35,840	34,392
Net Income	$ 3,220	$ 8,813	$ 5,919	$ 4,790	$ 4,560	$ 6,743	$ 5,089	$ 3,636	$ 22,742	$ 20,028	$ 18,445
Basic Earnings per Share	$ 0.20	$ 0.54	$ 0.36	$ 0.29	$ 0.29	$ 0.41	$ 0.31	$ 0.22	$ 1.39	$ 1.23	$ 1.14
Diluted Earnings per Share	$ 0.19	$ 0.54	$ 0.36	$ 0.29	$ 0.28	$ 0.41	$ 0.31	$ 0.22	$ 1.38	$ 1.22	$ 1.13