Stockholders' equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				12 Months Ended
	Sep. 23, 2016	Sep. 15, 2015	Mar. 12, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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				23,423,150	17,211,986
Common stock, shares outstanding				23,423,150	17,211,986
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
Aggregate net proceeds from public offering, net of underwriting commissions				$ 70,676	$ 91,793	$ 47,291
Underwriting commissions and offering costs				$ 4,692	$ 6,245	5,839
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				1,378,352	865,457
Stock options granted				658,710
Stock options granted, exercise price per share				$ 8.77
Common stock issued upon exercise of options				55,663
Common stock issued upon exercise of options aggregate intrinsic value				$ 277
Stock-based compensation expense				1,966	$ 1,506	$ 854
Unrecognized stock-based compensation cost				$ 4,574	$ 3,180
Unrecognized stock-based compensation cost, weighted-average period recognized				2 years 9 months 26 days	2 years 6 months 29 days
Common Stock [Member]
Equity Transactions And Share Based Compensation [Line Items]
Common stock, shares authorized				50,000,000	50,000,000
Common stock, par value				$ 0.000001
Common stock, shares issued				23,423,150	17,211,986
Common stock, shares outstanding				23,423,150	17,211,986	10,695,108	301,745
Sale of shares under public offering	6,152,500	6,325,000		6,152,500	6,325,000	4,830,000
Sale of shares under public offering, price per share	$ 12.25	$ 15.50
Aggregate net proceeds from public offering, net of underwriting commissions	$ 70,700	$ 91,800
Underwriting commissions and offering costs	$ 4,700	$ 6,200
Common stock issued upon exercise of options				55,663	191,878	86
IPO [Member] | Common Stock [Member]
Equity Transactions And Share Based Compensation [Line Items]
Sale of shares under public offering			4,830,000
Sale of shares under public offering, price per share			$ 11.00
Aggregate net proceeds from public offering, net of underwriting commissions			$ 47,300
Underwriting commissions and offering costs			$ 5,800
Employees [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted				621,210
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] | Minimum [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted, exercise price per share				$ 7.73
Employees [Member] | Maximum [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted, exercise price per share				$ 11.77
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
Director [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted				37,500
Stock options granted, exercise price per share				$ 7.73
Stock options, vesting period				1 year
Equity Incentive Plan 2006 [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options outstanding				338,935
Equity Incentive Plan 2014 [Member]
Equity Transactions And Share Based Compensation [Line Items]
Maximum number of shares of common stock that may be issued				1,953,137
Shares available to be granted				913,720	793,799
Stock options, expiration period				10 years