Quarterly Results of Operations (Unaudited) - Schedule of Selected Financial Data for Each Quarter (Parenthetical) (Detail) - $ / shares		3 Months Ended								12 Months Ended
	Jul. 29, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information [Line Items]
Sum of net loss per share calculated on quarterly basis										$ (1.91)
Loss per share - Basic and Diluted		$ (0.43)	$ (0.38)	$ (0.29)	$ (0.81)	$ (0.81)	$ (0.42)	$ (0.48)	$ (0.39)	$ (1.86)	$ (2.10)	$ (1.00)
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	3,763,440									5,117,500		4,327,956