Quarterly Results of Operations (Unaudited) - Schedule of Selected Financial Data for Each Quarter (Parenthetical) (Detail) - $ / shares			3 Months Ended								12 Months Ended
	Jul. 18, 2018	Mar. 30, 2017	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 [Line Items]
Sum of net loss per share calculated on quarterly basis												$ (1.91)
Loss per share - Basic and Diluted			$ (0.52)	$ (0.51)	$ (0.52)	$ (0.51)	$ (0.43)	$ (0.38)	$ (0.29)	$ (0.81)	$ (2.06)	$ (1.86)	$ (2.10)
Common Stock [Member]
Quarterly Financial Information [Line Items]
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Shares	5,175,000
Common Stock [Member] | Follow-on Public Offering [Member]
Quarterly Financial Information [Line Items]
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Shares		5,117,500									5,175,000	5,117,500