Selected Quarterly Financial Data (Unaudited) - Schedule of Selected Quarterly Financial Information (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Data [Abstract]
Total operating expenses	$ 17,267,000	$ 16,766,000	$ 13,322,000	$ 14,054,000	$ 13,116,000	$ 13,176,000	$ 13,210,000	$ 13,430,000	$ 61,409,467	$ 52,931,683
Total interest income	299,000	131,000	265,000	253,000	216,000	214,000	275,000	247,000	948,224	952,073
Net loss	(16,968,000)	(16,635,000)	(13,057,000)	(13,801,000)	(12,900,000)	(12,962,000)	(12,935,000)	(13,183,000)	(60,461,243)	(51,979,610)
Net loss applicable to common stockholders	$ (16,968,000)	$ (16,635,000)	$ (13,057,000)	$ (13,801,000)	$ (12,900,000)	$ (12,962,000)	$ (12,935,000)	$ (13,183,000)	$ (60,461,243)	$ (51,979,610)
Net loss per share applicable to common stockholders - basic and diluted	$ (0.35)	$ (0.43)	$ (0.34)	$ (0.46)	$ (0.52)	$ (0.53)	$ (0.53)	$ (0.54)