Common stock (Details)	12 Months Ended
	Dec. 31, 2020 Vote $ / shares shares	Jan. 31, 2021 shares	Dec. 31, 2019 $ / shares shares	Oct. 31, 2015 shares
Class of Stock [Line Items]
Common Stock, Shares Authorized	120,000,000		120,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.001		$ 0.001
Number of votes entitled for each share of common stock | Vote	1
Dividends declared | $ / shares	$ 0
Dividends paid | $ / shares	$ 0
Shares available for future issuance (in shares)	10,592,520		9,043,155
2015 Stock Option Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	2,827,185	7,707,888		1,311,812
Vesting of restricted stock awards | Founder Awards
Class of Stock [Line Items]
Shares available for future issuance (in shares)	156,863		176,471
Exercise of stock options
Class of Stock [Line Items]
Shares available for future issuance (in shares)	5,485,078		5,317,326
Vesting of restricted stock units
Class of Stock [Line Items]
Shares available for future issuance (in shares)	638,471		455,404
Issuance under share based compensation plans | 2015 Stock Option Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	2,827,185		1,875,078
Issuance under share based compensation plans | 2015 Employee Stock Purchase Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	1,484,923		1,218,876