Stock-based compensation - Stock Options and RSUs (Details)	6 Months Ended
	Jun. 30, 2019 $ / shares shares
Employee stock options
Number of Shares
Beginning Balance (in shares) | shares	3,202,745
Granted (in shares) | shares	84,336
Exercised/Converted (in shares) | shares	(166,071)
Cancelled/Forfeited (in shares) | shares	(158,011)
Ending Balance (in shares) | shares	2,962,999
Weighted Average Exercise Price
Beginning Balance (in USD per share) | $ / shares	$ 5.73
Granted (in USD per share) | $ / shares	4.33
Exercised/Converted (in USD per share) | $ / shares	3.45
Cancelled/Forfeited (in USD per share) | $ / shares	6.16
Ending Balance (in USD per share) | $ / shares	$ 5.80
Restricted stock units
Number of Units
Beginning Balance (in shares) | shares	2,486,028
Granted (in shares) | shares	332,034
Exercised/Converted (in shares) | shares	(423,229)
Cancelled/Forfeited (in shares) | shares	(186,353)
Ending Balance (in shares) | shares	2,208,480
Weighted Average Exercise Price
Beginning Balance (in USD per share) | $ / shares	$ 7.87
Granted (in USD per share) | $ / shares	5.28
Exercised/Converted (in USD per share) | $ / shares	7.12
Cancelled/Forfeited (in USD per share) | $ / shares	7.73
Ending Balance (in USD per share) | $ / shares	$ 7.63