12. Quarterly Financial Data (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 Information Disclosure [Abstract]
Operating Revenue	$ 34,024	$ 37,903	$ 36,164	$ 38,504	$ 33,294	$ 37,835	$ 36,143	$ 36,215	$ 146,595	$ 143,487	$ 145,733
Operating Income	8,293	11,479	10,175	11,327	7,611	10,976	10,074	10,465	41,274	39,126	42,510
Net Income	$ 8,646	$ 7,971	$ 10,026	$ 9,950	$ 5,571	$ 7,614	$ 7,450	$ 6,945	$ 36,593	$ 27,581	$ 28,925
Income Per Basic Share	$ 4.67	$ 4.3	$ 5.44	$ 5.42	$ 3.05	$ 4.17	$ 4.09	$ 3.81	$ 19.82	$ 15.12	$ 15.67
Income Per Diluted Share	$ 4.66	$ 4.29	$ 5.40	$ 5.36	$ 3.00	$ 4.10	$ 4.02	$ 3.76	$ 19.71	$ 14.85	$ 15.47