Stock-based compensation - Stock Options and RSUs (Details)	9 Months Ended
	Sep. 30, 2019 $ / shares shares
Employee stock options
Number of Shares
Beginning Balance (in shares) | shares	3,202,745
Granted (in shares) | shares	103,001
Exercised/Converted (in shares) | shares	(264,857)
Cancelled/Forfeited (in shares) | shares	(208,277)
Ending Balance (in shares) | shares	2,832,612
Weighted Average Exercise Price
Beginning Balance (in USD per share) | $ / shares	$ 5.73
Granted (in USD per share) | $ / shares	4.34
Exercised/Converted (in USD per share) | $ / shares	3.66
Cancelled/Forfeited (in USD per share) | $ / shares	6.52
Ending Balance (in USD per share) | $ / shares	$ 5.82
Restricted stock units
Number of Units
Beginning Balance (in shares) | shares	2,486,028
Granted (in shares) | shares	1,938,591
Exercised/Converted (in shares) | shares	(996,340)
Cancelled/Forfeited (in shares) | shares	(220,524)
Ending Balance (in shares) | shares	3,207,755
Weighted Average Exercise Price
Beginning Balance (in USD per share) | $ / shares	$ 7.87
Granted (in USD per share) | $ / shares	4.87
Exercised/Converted (in USD per share) | $ / shares	8.70
Cancelled/Forfeited (in USD per share) | $ / shares	7.51
Ending Balance (in USD per share) | $ / shares	$ 5.82