Stockholders' equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
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	23,464,930		23,464,930		23,423,150
Common stock, shares outstanding	23,464,930		23,464,930		23,423,150
Preferred stock, shares issued	0		0		0
Preferred stock, shares outstanding	0		0		0
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			796,790
Stock options granted, exercise price per share			$ 16.92
Common stock issued upon exercise of options	218		41,780
Stock-based compensation expense	$ 1.2	$ 0.5	$ 2.9	$ 1.4
Unrecognized stock-based compensation cost	$ 11.6	$ 4.4	$ 11.6	$ 4.4
Unrecognized stock-based compensation cost, weighted-average period recognized			2 years 10 months 17 days	2 years 10 months 21 days
Employees [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted			715,790
Employees [Member] | Minimum [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted, exercise price per share			$ 13.74
Employees [Member] | Maximum [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted, exercise price per share			$ 17.35
Employees [Member] | One Year after the Beginning of the Vesting Period [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options, vesting percentage			25.00%
Employees [Member] | Condition (i) [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options, vesting period			1 year
Employees [Member] | Condition (ii) [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options, vesting period			3 years
Non-Employee Directors [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted			81,000
Stock options granted, exercise price per share			$ 13.74
Non-Employee Directors [Member] | Condition (i) [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options, vesting period			1 year
Non-Employee Directors [Member] | Condition (ii) [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options, vesting period			3 years
Equity Incentive Plan 2014 [Member]
Equity Transactions And Share Based Compensation [Line Items]
Maximum number of shares of common stock that may be issued	2,863,908		2,863,908
Stock options, expiration period			10 years