Selected Interim Financial Data (unaudited) - Summary of Unaudited Quarterly Financial Information (Details) - 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]
Revenues	$ 13,306	$ 9,678	$ 17,560	$ 16,716	$ 16,232	$ 15,766	$ 14,825	$ 14,282	$ 57,260	$ 61,105	$ 49,296
Interest expense	(3,953)	(3,190)	(2,786)	(2,558)	(2,261)	(2,117)	(1,808)	(1,515)	(12,487)	(7,701)	(10,883)
Net income (loss)	362	(1,196)	4,186	6,154	6,679	6,266	5,732	5,475	9,506	24,152	11,316
Net income attributable to noncontrolling interest	(953)	(951)	(954)	(985)	(909)	(941)	(936)	(944)	(3,843)	(3,730)	(266)
Net income (loss) attributable to common stockholders	$ (591)	$ (2,147)	$ 3,232	$ 5,169	$ 5,770	$ 5,325	$ 4,796	$ 4,531	$ 5,663	$ 20,422	$ (2,710)
Net income (loss) attributable to common stockholders per share:
Basic and diluted	$ (0.02)	$ (0.07)	$ 0.10	$ 0.16	$ 0.18	$ 0.17	$ 0.15	$ 0.14	$ 0.17	$ 0.64	$ (0.18)