Stockholders' equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Sep. 23, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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		23,537,368	23,472,430
Common stock, shares outstanding		23,537,368	23,472,430
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
Aggregate net proceeds from public offering, net of underwriting commissions				70,676
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		2,897,294	2,069,167
Stock options granted		2,056,640
Stock options granted, exercise price per share		$ 9.09
Common stock issued upon exercise of options		64,938
Common stock issued upon exercise of options aggregate intrinsic value		$ 300
Stock-based compensation expense		4,700	$ 3,800	$ 2,000
Unrecognized stock-based compensation cost		$ 7,800	$ 10,100
Unrecognized stock-based compensation cost, weighted-average period recognized		2 years 9 months 14 days	2 years 8 months 19 days
Common Stock [Member]
Equity Transactions And Share Based Compensation [Line Items]
Common stock, shares outstanding		23,537,368	23,472,430	23,423,150	17,211,986
Sale of shares under public offering	6,152,500			6,152,500
Sale of shares under public offering, price per share	$ 12.25
Underwriting commissions and offering costs	$ 4,700
Aggregate net proceeds from public offering, net of underwriting commissions	$ 70,700
Common stock issued upon exercise of options		64,938	49,280	55,663
Equity Incentive Plan 2014 [Member]
Equity Transactions And Share Based Compensation [Line Items]
Maximum number of shares of common stock that may be issued		3,746,778
Shares available to be granted		1,156,378	1,110,546
Equity Incentive Plan 2006 [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options outstanding		306,894
Employees [Member]
Equity Transactions And Share Based Compensation [Line Items]
Stock options granted		1,951,640
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		$ 2.61
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