Stock-based compensation (Details) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended				61 Months Ended
	Dec. 31, 2019 $ / shares shares	Oct. 31, 2015 shares	Jan. 31, 2014 $ / shares shares	Dec. 31, 2020 USD ($) item $ / shares shares	Dec. 31, 2019 USD ($) item $ / shares shares	Dec. 31, 2018 USD ($) item shares	Dec. 31, 2016 item	Jan. 31, 2021 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)	9,043,155			10,592,520	9,043,155
Stock-based compensation. | $				$ 14,934	$ 15,640	$ 15,710
Stock options granted (in shares)				1,560,575
Exercise price | $ / shares	$ 15.98			$ 14.77	$ 15.98
2014 Stock Option and Grant Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for stock-based compensation plan (in shares)			823,529
2015 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)				1,484,923				1,926,969
Percentage of outstanding common stock								1.00%
New stock options issued				104,108	70,217
Number of shares authorized for stock-based compensation plan (in shares)		262,362
Period to purchase shares				6 months
2015 Employee Stock Purchase Plan | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of payroll deduction		1
2015 Employee Stock Purchase Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of payroll deduction		10
2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)		1,311,812		2,827,185				7,707,888
Percentage limit of issued and outstanding shares up to which additional shares may be issued		4.00%
Percentage of outstanding common stock								4.00%
Outstanding stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation. | $				$ 11,387	$ 13,380	$ 14,956
Outstanding stock options | Inducement Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued				29,000	58,125	650,000
Number of executives issued awards | item				3	2	1
Vesting period				4 years
Outstanding stock options | Inducement Awards | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage				25.00%
Outstanding stock options | Inducement Awards | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				36 months
Vesting percentage				75.00%
Unvested restricted common stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation. | $				$ 3,110	$ 1,935	$ 482
Unrecognized stock-based compensation expense | $				$ 5,600
Vesting period				3 years
Restricted stock granted, original issued price (in dollars per share) | $ / shares				$ 12.67
Unvested restricted common stock units | Inducement Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				3 years
Unvested restricted common stock units | Inducement Awards | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage				33.00%
Unvested restricted common stock units | Inducement Awards | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage				33.00%
Performance based milestone awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Probable performance milestones | item							1
Performance milestones | item							3
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	58,823				58,823
Stock-based compensation. | $						300
Vesting period	3 years
Founders restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)			1,188,233
Restricted stock granted, original issued price (in dollars per share) | $ / shares			$ 0.0425
Founders restricted stock | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)			835,292
Founders restricted stock | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock granted (in shares)			352,941
Founders restricted stock | Minimum | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			1 year
Founders restricted stock | Maximum | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			4 years
Non-statutory stock options | Inducement Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New stock options issued				172,500	338,750
Employees and directors stock options | 2014 Stock Option and Grant Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				4 years
Employees and directors stock options | 2014 Stock Option and Grant Plan | Vesting period, tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				1 year
Vesting percentage				25.00%
Employees and directors stock options | 2014 Stock Option and Grant Plan | Vesting period, tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				3 years
Vesting percentage				75.00%
Employees and directors stock options | 2015 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)				1,388,075
Non-employees stock options | 2014 Stock Option and Grant Plan | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				1 year
Non-employees stock options | 2014 Stock Option and Grant Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				4 years
Share-based Payment Arrangement, Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation. | $				$ 11,200	$ 13,300	14,600
Share-based Payment Arrangement, Employee | Unvested restricted common stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation. | $				2,800	1,900	100
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation. | $				6,357	7,383	4,717
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation. | $				$ 8,577	$ 8,257	$ 10,993