14. 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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 108,183	$ 108,516	$ 104,933	$ 100,649	$ 95,308	$ 93,991	$ 88,361	$ 85,989	$ 83,467	$ 77,050	$ 71,594	$ 68,146	$ 422,282	$ 363,650	$ 300,256
Income before income tax expense	12,651	12,455	12,325	12,229	15,783	15,649	15,200	14,749	14,346	13,804	12,329	11,764	49,661	61,382	52,242
Net income	$ 7,465	$ 7,348	$ 7,272	$ 7,214	$ 8,967	$ 8,843	$ 8,537	$ 8,333	$ 8,010	$ 7,776	$ 7,026	$ 6,705	$ 29,300	$ 34,681	$ 29,516
Earnings per share - basic	$ .31	$ .31	$ .30	$ .29	$ 0.35	$ 0.34	$ 0.33	$ 0.33	$ 0.31	$ 0.31	$ 0.28	$ 0.28	$ 1.20	$ 1.34	$ 1.18
Earnings per share - diluted	$ .26	$ .26	$ .25	$ .24	$ 0.29	$ 0.28	$ 0.27	$ 0.26	$ 0.25	$ 0.24	$ 0.22	$ 0.21	$ 1.01	$ 1.10	$ 0.92