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]
Total revenues	$ 33,914	$ 33,492	$ 33,756	$ 32,623	$ 31,092	$ 31,777	$ 31,023	$ 29,129	$ 133,785	$ 123,021	$ 130,936
Income (loss) before income taxes	9,788	5,752	10,583	7,770	10,799	9,053	8,512	5,646	33,893	34,010	42,220
Net income	$ 2,897	$ 4,132	$ 6,896	$ 6,064	$ 7,577	$ 5,887	$ 5,661	$ 3,522	$ 19,989	$ 22,647	$ 27,105
Basic earnings per common share (in dollars per share)	$ 0.35	$ 0.51	$ 0.84	$ 0.75	$ 0.95	$ 0.74	$ 0.71	$ 0.45	$ 2.45	$ 2.84	$ 3.49
Diluted earnings per common share (in dollars per share)	$ 0.34	$ 0.49	$ 0.83	$ 0.73	$ 0.92	$ 0.72	$ 0.69	$ 0.44	$ 2.38	$ 2.77	$ 3.33