12. Quarterly Financial Data (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 Information Disclosure [Abstract]
Operating Revenue	$ 32,372	$ 37,381	$ 37,655	$ 38,324	$ 32,693	$ 36,625	$ 35,025	$ 36,419	$ 145,733	$ 140,762	$ 131,993
Operating Income	8,330	11,573	11,120	11,486	8,632	11,226	10,257	10,700	42,510	40,817	37,944
Net Income	$ 6,050	$ 7,799	$ 7,474	$ 7,602	$ 6,040	$ 7,685	$ 6,882	$ 7,201	$ 28,925	$ 27,808	$ 26,582
Income Per Basic Share	$ 3.32	$ 4.25	$ 4.04	$ 4.05	$ 3.11	$ 3.94	$ 3.51	$ 3.63	$ 15.67	$ 14.20	$ 13.22
Income Per Diluted Share	$ 3.27	$ 4.19	$ 3.99	$ 4.01	$ 3.08	$ 3.91	$ 3.48	$ 3.61	$ 15.47	$ 14.08	$ 13.18