Stockholders' equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended
	Sep. 15, 2015	Jan. 27, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [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			17,211,986		17,211,986		10,695,108
Common stock, shares outstanding			17,211,986		17,211,986		10,695,108
Preferred stock, shares issued			0		0		0
Preferred stock, shares outstanding			0	0	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			0		287,750
Stock options granted, exercise price per share					$ 11.46
Stock options, vesting period					6 years 4 months 13 days
Common stock issued upon exercise of options					191,878
Stock-based compensation expense			$ 424	$ 233	$ 1,145	$ 585
Unrecognized stock-based compensation cost			$ 3,513	$ 2,982	$ 3,513	$ 2,982
Unrecognized stock-based compensation cost, weighted-average period recognized					2 years 9 months 4 days	2 years 10 months 13 days
Gross proceeds in public offering before underwriting commissions and expenses					$ 98,038	$ 53,130
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued under public offering	6,325,000
Shares issued under public offering, price per share	$ 15.5
Gross proceeds in public offering before underwriting commissions and expenses	$ 98,000
Underwriting commissions and expenses	$ 6,200
Common Stock [Member] | Underwriters [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued under public offering	825,000
Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted					257,750
Employees [Member] | Vesting of stock options granted to employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, vesting percentage					25.00%
Employees [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted, exercise price per share					$ 7.45
Employees [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted, exercise price per share					$ 12.03
Director [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted					30,000
Non-employee Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted, exercise price per share					$ 12.03
Non-employee Directors [Member] | Condition (i) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, vesting period					1 year
Non-employee Directors [Member] | Condition (ii) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, vesting period					3 years
Equity Incentive Plan 2006 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted		0
Equity Incentive Plan 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of shares of common stock that may be issued			1,271,756		1,271,756
Stock options, expiration period					10 years