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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 98,583	$ 84,890	$ 96,353	$ 77,749	$ 90,377	$ 80,369	$ 89,733	$ 77,194	$ 357,575	$ 337,673	$ 326,095
Income (loss) from operations	8,744	4,942	11,027	(2,288)	7,451	6,749	10,213	3,136	22,425	27,549	30,774
Net income (loss)	$ 5,375	$ 3,950	$ 7,788	$ (1,733)	$ 2,093	$ 3,953	$ 6,064	$ 3,030	$ 15,380	$ 15,140	$ 17,651
Basic income (loss) per common share	$ 0.29	$ 0.22	$ 0.43	$ (0.10)	$ 0.12	$ 0.22	$ 0.34	$ 0.17	$ 0.85	$ 0.84	$ 0.98
Diluted income (loss) per common share	$ 0.29	$ 0.21	$ 0.43	$ (0.10)	$ 0.11	$ 0.22	$ 0.34	$ 0.16	$ 0.84	$ 0.83	$ 0.97