Stockholders' equity - Additional Information (Detail) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 08, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
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		28,140,243		28,140,243		23,537,368
Common stock, shares outstanding		28,140,243		28,140,243		23,537,368
Preferred stock, shares issued		101,927		101,927		0
Preferred stock, shares outstanding		101,927		101,927		0
Maximum number of shares of common stock that may be issued		4,712,447		4,712,447
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
Options granted, Number of Shares				298,000
Common stock issued upon exercise of options		13,104		13,104
Stock-based compensation expense		$ 5,100,000	$ 1,500,000	$ 7,000,000	$ 2,800,000
Unrecognized stock-based compensation cost		8,600,000	$ 15,400,000	$ 8,600,000	$ 15,400,000
Unrecognized stock-based compensation cost, weighted-average period recognized				3 years 8 months 12 days	3 years
Stock conversion ratio	1 preferred stock = 100 common stock
Preferred stock, liquidation	$ 1
Stock options granted, exercise price per share				$ 3.54
Common stock issued upon exercise of options aggregate intrinsic value		$ 0
Weighted average grant-date fair value of stock options		$ 2.63	$ 2.20	$ 2.63	$ 6.25
Stock options outstanding weighted average remaining contractual life				3 years 11 months 12 days		7 years 11 months 15 days
Stock options outstanding intrinsic value		$ 3,000		$ 3,000
First year [Member] | Restricted Stock Units (RSUs) [Member]
Vesting percentage		50.00%
Second year [Member] | Restricted Stock Units (RSUs) [Member]
Vesting percentage		50.00%
2014 Plan | Restricted Stock Units (RSUs) [Member]
Award expiration period		10 years
Employees And Consultants [Member]
Options granted, Number of Shares		298,000
Employees And Consultants [Member] | Restricted Stock Units (RSUs) [Member]
Restricted shares Granted		72,000
Weighted average Fair value Grant		$ 3.47
Employees [Member]
Vesting percentage		25.00%
Stock options, vesting period		36 months
Employees [Member] | Inducement Grants [Member]
Options granted, Number of Shares		3,300,000
Stock options granted, exercise price per share		$ 2.80
Vesting percentage		25.00%
Stock options, vesting period		36 months
Weighted average grant-date fair value of stock options		$ 2.08
Stock options outstanding weighted average remaining contractual life		9 years 11 months 1 day
Stock options outstanding intrinsic value		$ 200,000		$ 200,000
Employees [Member] | 2014 Plan
Award expiration period		10 years
Employees [Member] | Minimum [Member]
Stock options granted, exercise price per share		$ 2.80
Employees [Member] | Maximum [Member]
Stock options granted, exercise price per share		$ 3.76
Common Stock [Member]
Stock issued for acquisition		4,589,771		4,589,771
Common stock issued upon exercise of options		13,104		13,104	64,938
Aquinox [Member] | Common Stock [Member]
Stock issued for acquisition	4,589,771
Non-voting convertible preferred stock [Member]
Preferred stock, shares authorized		5,000,000		5,000,000		5,000,000
Preferred stock, par value		$ 0.000001		$ 0.000001		$ 0.000001
Preferred stock, shares issued		101,927		101,927		0
Preferred stock, shares outstanding		101,927		101,927		0
Non-voting convertible preferred stock [Member] | Aquinox [Member]
Stock issued for acquisition	101,927