14. 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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 95,308	$ 93,991	$ 88,361	$ 85,989	$ 83,467	$ 77,050	$ 71,594	$ 68,146	$ 66,634	$ 64,066	$ 70,482	$ 54,594	$ 363,650	$ 300,256	$ 255,775
Income before income tax expense	15,783	15,649	15,200	14,749	14,346	13,804	12,329	11,764	11,540	10,559	8,546	6,528	61,382	52,242	37,173
Net income	$ 8,967	$ 8,843	$ 8,537	$ 8,333	$ 8,010	$ 7,776	$ 7,026	$ 6,705	$ 6,522	$ 5,873	$ 4,825	$ 3,785	$ 34,681	$ 29,516	$ 21,005
Earnings per share - basic	$ .35	$ .34	$ .33	$ .33	$ 0.31	$ 0.31	$ 0.28	$ 0.28	$ 0.28	$ 0.27	$ 0.23	$ 0.19	$ 1.34	$ 1.18	$ 0.98
Earnings per share - diluted	$ .29	$ .28	$ .27	$ .26	$ 0.25	$ 0.24	$ 0.22	$ 0.21	$ 0.21	$ 0.19	$ 0.15	$ 0.12	$ 1.10	$ 0.92	$ 0.67