Share-Based Payments - Summary of Stock Option Activity (Detail)	9 Months Ended
	Sep. 30, 2018 $ / shares shares
Shares
Outstanding at beginning of period (in shares) | shares	2,695,796
Granted (in shares) | shares	3,055,900
Exercised (in shares) | shares	(71,403)
Cancelled or forfeited (in shares) | shares	(1,111,941)
Outstanding at end of period (in shares) | shares	4,568,352
Exercisable at end of period (in shares) | shares	1,868,379
Weighted-Average Exercise Price
Outstanding at beginning of period (in USD per share) | $ / shares	$ 3.16
Granted (in USD per share) | $ / shares	1.72
Exercised (in USD per share) | $ / shares	0.79
Cancelled or forfeited (in USD per share) | $ / shares	2.40
Outstanding at end of period (in USD per share) | $ / shares	2.42
Exercisable at end of period (in USD per share) | $ / shares	$ 3.35