Stockholders' equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Sep. 23, 2016	Sep. 15, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity Transactions And Share Based Compensation [Line Items]
Common stock, shares authorized			50,000,000	50,000,000
Common stock, par value			$ 0.000001	$ 0.000001
Common stock, shares issued			23,472,430	23,423,150
Common stock, shares outstanding			23,472,430	23,423,150
Preferred stock, shares authorized			5,000,000	5,000,000
Preferred stock, par value			$ 0.000001	$ 0.000001
Preferred stock, shares issued			0	0
Preferred stock, shares outstanding			0	0
Underwriting commissions and offering costs				$ 4,692	$ 6,245
Aggregate net proceeds from public offering, net of underwriting commissions				$ 70,676	91,793
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 stock 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 outstanding			2,069,167	1,378,352
Stock options granted			837,540
Stock options granted, exercise price per share			$ 16.63
Common stock issued upon exercise of options			49,280
Common stock issued upon exercise of options aggregate intrinsic value			$ 200
Stock-based compensation expense			3,800	$ 2,000	$ 1,500
Unrecognized stock-based compensation cost			$ 10,100	$ 4,600
Unrecognized stock-based compensation cost, weighted-average period recognized			2 years 8 months 19 days	2 years 9 months 25 days
Common Stock [Member]
Equity Transactions And Share Based Compensation [Line Items]
Common stock, shares outstanding			23,472,430	23,423,150	17,211,986	10,695,108
Sale of shares under public offering	6,152,500	6,325,000		6,152,500	6,325,000
Sale of shares under public offering, price per share	$ 12.25	$ 15.50
Underwriting commissions and offering costs	$ 4,700	$ 6,200
Aggregate net proceeds from public offering, net of underwriting commissions	$ 70,700	$ 91,800
Common stock issued upon exercise of options			49,280	55,663	191,878
Employees [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted			756,540
Employees [Member] | Minimum [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted, exercise price per share			$ 10.91
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			2,856,403
Shares available to be granted			1,110,546	913,720
Stock options, expiration period			10 years
Equity Incentive Plan 2006 [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options outstanding			323,310