Net Loss Per Share - Schedule of Computation of Basic and Diluted Net Loss Per Share (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net loss	$ (29,905,237)	$ (4,795,059)	$ (37,594,401)	$ (8,024,985)
Less: preferred stock accretion and dividends	(1,962,072)	(104,548)	(2,125,141)	(247,562)
Net loss attributed to common stockholders	$ (31,867,309)	$ (4,899,607)	$ (39,719,542)	$ (8,272,547)
Denominator:
Weighted-average common shares outstanding, basic and diluted	15,527,867	6,911,715	11,595,698	6,911,366
Net loss per share attributed to common stockholders, basic and diluted	$ (2.05)	$ (0.71)	$ (3.43)	$ (1.20)