7. Income Per Share (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
Earnings Per Share [Abstract]
Net Income	$ 8,646	$ 7,971	$ 10,026	$ 9,950	$ 5,571	$ 7,614	$ 7,450	$ 6,945	$ 36,593	$ 27,581	$ 28,925
Weighted Average Basic Shares Outstanding									1,846	1,824	1,846
Add: Effect of dilutive securities									11	33	24
Weighted average diluted shares outstanding									1,857	1,857	1,870
Net Income per share
Basic	$ 4.67	$ 4.3	$ 5.44	$ 5.42	$ 3.05	$ 4.17	$ 4.09	$ 3.81	$ 19.82	$ 15.12	$ 15.67
Diluted	$ 4.66	$ 4.29	$ 5.40	$ 5.36	$ 3.00	$ 4.10	$ 4.02	$ 3.76	$ 19.71	$ 14.85	$ 15.47