Summary Selected Quarterly Financial Data - Summary of Selected Consolidated Quarterly Financial Data (unaudited) (Detail) - 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	$ 90,377	$ 80,369	$ 89,733	$ 77,194	$ 83,446	$ 77,427	$ 87,821	$ 77,401	$ 337,673	$ 326,095	$ 313,726
Income from operations	7,451	6,749	10,213	3,136	9,038	7,552	11,472	2,712	27,549	30,774	18,827
Net income	$ 2,093	$ 3,953	$ 6,064	$ 3,030	$ 5,790	$ 3,112	$ 7,460	$ 1,289	$ 15,140	$ 17,651	$ 11,996
Basic income per common share	$ 0.12	$ 0.22	$ 0.34	$ 0.17	$ 0.32	$ 0.17	$ 0.42	$ 0.07	$ 0.84	$ 0.98	$ 0.67
Diluted income per common share	$ 0.11	$ 0.22	$ 0.34	$ 0.16	$ 0.31	$ 0.17	$ 0.41	$ 0.07	$ 0.83	$ 0.97	$ 0.66