Stock-based compensation - Stock Options and RSUs (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee stock options
Shares Available for Grant
Beginning Balance (in shares)	1,102,323
Authorized for issuance (in shares)	1,623,244
Granted (in shares)	(2,087,020)
Exercised/Converted (in shares)	0
Cancelled/Forfeited (in shares)	(670,668)
Ending Balance (in shares)	1,309,215	1,102,323
Number of Shares
Beginning Balance (in shares)	3,202,745
Authorized for issuance (in shares)	0
Granted (in shares)	103,001
Exercised/Converted (in shares)	(485,832)
Cancelled/Forfeited (in shares)	(221,169)
Ending Balance (in shares)	2,598,745	3,202,745
Weighted Average Exercise Price
Beginning Balance (USD per share)	$ 5.73
Authorized for issuance (in shares)	0
Granted (USD per share)	4.34
Exercised/Converted (USD per share)	3.90
Cancelled/Forfeited (USD per share)	6.81
Ending Balance (USD per share)	$ 5.93	$ 5.73
Restricted stock units
Number of Units
Beginning Balance (in shares)	2,486,028
Authorized for issuance (in shares)	0
Granted (in shares)	1,974,019
Exercised/Converted (in shares)	(1,018,143)
Cancelled/Forfeited (in shares)	(270,923)
Ending Balance (in shares)	3,170,981	2,486,028
Weighted Average Exercise Price
Beginning Balance (USD per share)	$ 7.87
Authorized for issuance (in shares)	0
Granted (USD per share)	4.90	$ 6.83	$ 7.86
Exercised/Converted (USD per share)	8.72
Cancelled/Forfeited (USD per share)	7.23
Ending Balance (USD per share)	$ 5.80	$ 7.87