12. Quarterly Financial Data (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 Revenue	$ 33,294	$ 37,835	$ 36,143	$ 36,215	$ 32,372	$ 37,381	$ 37,655	$ 38,324	$ 143,487	$ 145,733	$ 140,762
Operating Income	7,611	10,976	10,074	10,465	8,330	11,573	11,120	11,486	39,126	42,510	40,817
Net Income	$ 5,571	$ 7,614	$ 7,450	$ 6,945	$ 6,050	$ 7,799	$ 7,474	$ 7,602	$ 27,581	$ 28,925	$ 27,808
Income Per Basic Share	$ 3.05	$ 4.17	$ 4.09	$ 3.81	$ 3.32	$ 4.25	$ 4.04	$ 4.05	$ 15.12	$ 15.67	$ 14.20
Income Per Diluted Share	$ 3.00	$ 4.10	$ 4.02	$ 3.76	$ 3.27	$ 4.19	$ 3.99	$ 4.01	$ 14.85	$ 15.47	$ 14.08