Common stock (Details)	12 Months Ended
	Dec. 31, 2018 Vote $ / shares shares	Dec. 31, 2017 $ / 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)	7,397,059	5,933,410
2015 Stock Option Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	1,973,227		1,311,812
Vesting of restricted stock awards | Founder Awards
Class of Stock [Line Items]
Shares available for future issuance (in shares)	235,294	366,914
Vesting of restricted stock awards | 2014 Stock Option and Stock Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)		191,065
Exercise of stock options
Class of Stock [Line Items]
Shares available for future issuance (in shares)	4,225,152	3,143,566
Issuance under share based compensation plans
Class of Stock [Line Items]
Shares available for future issuance (in shares)	963,386	730,860
Issuance under share based compensation plans | 2015 Stock Option Plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)	1,973,227	1,501,005