Quarterly Financial Information (unaudited) (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 Data [Abstract]
Total income	$ 13,797	$ 14,226	$ 13,105	$ 13,667	$ 12,292	$ 12,106	$ 10,843	$ 11,456	$ 54,795	$ 46,697	$ 37,568
Income before provision for income tax	6,673	7,763	7,150	8,699	6,157	7,905	6,887	7,960	30,285	28,909	25,794
Net income attributable to common stockholders	$ 6,184	$ 7,470	$ 6,864	$ 8,409	$ 5,957	$ 7,623	$ 6,605	$ 7,651	$ 28,606	$ 27,696	$ 24,692
Basic earnings per common share (in dollars per share)	$ 0.34	$ 0.41	$ 0.38	$ 0.46	$ 0.33	$ 0.42	$ 0.42	$ 0.50	$ 1.58	$ 1.65	$ 1.68
Diluted earnings per common share (in dollars per share)	$ 0.33	$ 0.38	$ 0.36	$ 0.46	$ 0.33	$ 0.42	$ 0.42	$ 0.50	$ 1.51	$ 1.65	$ 1.68