7. Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net Income	$ 7,749	$ 9,221	$ 8,797	$ 8,487	$ 8,646	$ 7,971	$ 10,026	$ 9,950	$ 34,255	$ 36,593	$ 27,581
Weighted Average Basic Shares Outstanding									1,853	1,846	1,824
Add: Effect of dilutive securities									5	11	33
Weighted average diluted shares outstanding									1,858	1,857	1,857
Net Income per share
Basic	$ 4.18	$ 4.98	$ 4.75	$ 4.58	$ 4.67	$ 4.3	$ 5.44	$ 5.42	$ 18.49	$ 19.82	$ 15.12
Diluted	$ 4.17	$ 4.96	$ 4.74	$ 4.57	$ 4.66	$ 4.29	$ 5.40	$ 5.36	$ 18.44	$ 19.71	$ 14.85