Stockholders' equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
	Dec. 17, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Total number of shares authorized		105,000,000
Common stock, shares authorized		100,000,000		100,000,000
Common stock, par value		$ 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		38,386,160		37,996,849
Common stock, shares outstanding		38,386,160		37,996,849
Preferred stock, shares issued		0		0
Preferred stock, shares outstanding		0		0
Conversion of common stock to warrants	10,925,481
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.
Years of increase in shares of common stock reserved for issuance		10 years
Shares of common stock reserved for issuance percentage		4.00%
Options granted, Number of Shares		290,700
Options granted, Weighted Average Exercise Price		$ 7.63
Options exercised, shares		376,311
Common stock issued upon exercise of options aggregate intrinsic value		$ 1.3
Unrecognized stock-based compensation cost		$ 9.8	$ 6.7
Unrecognized stock-based compensation cost, weighted-average period recognized		3 years 4 months 9 days	2 years 7 months 9 days
Research and Development Expense [Member]
Stock-based compensation expense		$ 0.3	$ 0.1
General and Administrative Expense [Member]
Stock-based compensation expense		$ 0.4	$ 1.0
Restricted Stock Units (RSUs) [Member]
Restricted shares Granted		75,000
Restricted stock units granted, Weighted Average Grant Date Fair Value		$ 6.44
Shares issued upon vesting of units		13,000
Shares issued upon vesting of units aggregate intrinsic value		$ 0.1
Employees And Consultants [Member]
Options granted, Number of Shares		290,700
Employees [Member]
Vesting Percentage		25.00%
Employees [Member] | Restricted Stock Units (RSUs) [Member]
Restricted shares Granted		75,000
Restricted stock units granted, Weighted Average Grant Date Fair Value		$ 6.44
Employees [Member] | First Year [Member] | Restricted Stock Units (RSUs) [Member]
Vesting Percentage		33.33%
Employees [Member] | Second year [Member] | Restricted Stock Units (RSUs) [Member]
Vesting Percentage		33.33%
Employees [Member] | Third year [Member] | Restricted Stock Units (RSUs) [Member]
Vesting Percentage		33.33%
Employees [Member] | Minimum [Member]
Options granted, Weighted Average Exercise Price		$ 6.44
Employees [Member] | Maximum [Member]
Options granted, Weighted Average Exercise Price		$ 12.44
2014 Plan
Maximum number of shares of common stock that may be issued		10,273,506
2014 Plan | Employees [Member]
Award expiration period		10 years
2014 Plan | Employees [Member] | Restricted Stock Units (RSUs) [Member]
Award expiration period		0 years
Pre Funded Common Stock Warrants [Member]
Number of common stock in prefunded warrants	10,925,481
Exercise Price Per Share	$ 0.000001
Description of Covenants Under Exercise Of Warrants		The Exchange Warrants may be exercised at any time after the date of issuance, except that the Exchange Warrants cannot be exercised by the stockholders if, after giving effect thereto, the stockholders would beneficially own more than 9.99% of the outstanding common stock, subject to certain exceptions.