Stock-based compensation - Stock Options and RSUs (Details)	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Employee stock options
Number of Shares
Beginning Balance (in shares) | shares	4,301,340
Granted (in shares) | shares	502,746
Exercised/Converted (in shares) | shares	(515,988)
Cancelled/Forfeited (in shares) | shares	(61,106)
Ending Balance (in shares) | shares	4,226,992
Weighted Average Exercise Price
Beginning Balance (in USD per share) | $ / shares	$ 5.18
Granted (in USD per share) | $ / shares	7.37
Exercised/Converted (in USD per share) | $ / shares	3.80
Cancelled/Forfeited (in USD per share) | $ / shares	8.71
Ending Balance (in USD per share) | $ / shares	$ 5.56
Restricted stock units
Number of Units
Beginning Balance (in shares) | shares	2,089,473
Granted (in shares) | shares	1,351,071
Exercised/Converted (in shares) | shares	(789,123)
Cancelled/Forfeited (in shares) | shares	(147,783)
Ending Balance (in shares) | shares	2,503,638
Weighted Average Exercise Price
Beginning Balance (in USD per share) | $ / shares	$ 10.15
Granted (in USD per share) | $ / shares	7.91
Exercised/Converted (in USD per share) | $ / shares	9.40
Cancelled/Forfeited (in USD per share) | $ / shares	10.50
Ending Balance (in USD per share) | $ / shares	$ 9.15