Stockholders' equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Equity Transactions And Share Based Compensation [Line Items]
Total number of shares authorized	55,000,000		55,000,000
Common stock, shares authorized	50,000,000		50,000,000		50,000,000
Common stock, par value	$ 0.000001		$ 0.000001		$ 0.000001
Preferred stock, shares authorized	5,000,000		5,000,000
Preferred stock, par value	$ 0.000001		$ 0.000001
Common stock, shares issued	23,537,368		23,537,368		23,472,430
Common stock, shares outstanding	23,537,368		23,537,368		23,472,430
Preferred stock, shares issued	0		0		0
Preferred stock, shares outstanding	0		0		0
Maximum number of shares of common stock that may be issued	3,746,696		3,746,696
Shares of common stock reserved for issuance description			Additionally, the number of shares of common stock reserved for issuance under the 2014 Plan will automatically increase on January 1 of each year for a period of up to 10 years, beginning on January 1, 2015 and ending on and including January 1, 2024, by 4% of the total number of shares of capital outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the board of directors.
Shares of common stock reserved for issuance percentage			4.00%
Years of increase in shares of common stock reserved for issuance			10 years
Stock options granted			2,054,540
Stock options granted, exercise price per share			$ 9.10
Common stock issued upon exercise of options			64,938
Common stock issued upon exercise of options aggregate intrinsic value			$ 0.3
Stock-based compensation expense	$ 1.0	$ 1.2	3.9	$ 2.9
Unrecognized stock-based compensation cost	$ 12.1	$ 11.6	$ 12.1	$ 11.6
Unrecognized stock-based compensation cost, weighted-average period recognized			2 years 11 months 4 days	2 years 10 months 17 days
Employees [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted			1,949,540
Stock options, expiration period			10 years
Employees [Member] | Minimum [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted, exercise price per share			$ 3.07
Employees [Member] | Maximum [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted, exercise price per share			$ 16.55
Employees [Member] | Vesting 25% One Year After the Beginning of the Vesting Period and Monthly Thereafter [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options, vesting period from grant date			1 year
Stock options, vesting percentage			25.00%
Stock options, vesting period			36 months
Non-Employee Directors [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted			105,000
Stock options granted, exercise price per share			$ 13.10
Stock options, vesting period			1 year