Selected Quarterly Data - Summary of Quarterly Financial Information (Detail) - USD ($)	3 Months Ended					9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule Of Quarterly Financial Information [Line Items]
Total revenues	$ 20,313,069	$ 20,045,516	$ 19,866,457	$ 19,385,071	$ 19,939,512	$ 60,225,042	$ 56,723,834
Total operating expenses	15,836,436	16,605,713	16,010,116	16,991,318	19,197,777	48,452,265	55,542,477
Operating income	4,476,633	3,439,803	3,856,341	2,393,753	741,735	11,772,777	1,181,357
Net loss	(457,278)	(1,437,330)	(667,047)	(2,351,155)	(3,875,164)	(2,561,655)	(12,635,135)
Net loss attributable to common stockholders	$ (451,424)	$ (1,427,056)	$ (661,203)	$ (2,329,515)	$ (3,840,775)	$ (2,539,683)	$ (12,534,550)
Class A Common stock
Schedule Of Quarterly Financial Information [Line Items]
Net loss per share-basic and diluted	$ (0.01)	$ (0.02)	$ (0.01)	$ (0.04)	$ (0.07)	$ (0.04)	$ (0.22)
Class T Common stock
Schedule Of Quarterly Financial Information [Line Items]
Net loss per share-basic and diluted	$ (0.01)	$ (0.02)	$ (0.01)	$ (0.04)	$ (0.07)	$ (0.04)	$ (0.22)