Stock-based compensation - Stock Options and RSUs (Details)	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Employee Stock Options
Number of Shares
Beginning Balance | shares	4,301,340
Granted | shares	45,126
Exercised/Converted | shares	(402,977)
Cancelled/Forfeited | shares	(44,977)
Ending Balance | shares	3,898,512
Weighted Average Exercise Price
Beginning Balance | $ / shares	$ 5.18
Granted | $ / shares	9.20
Exercised/Converted | $ / shares	3.76
Cancelled/Forfeited | $ / shares	9.15
Ending Balance | $ / shares	$ 5.33
Restricted stock units
Number of Units
Beginning Balance | shares	2,089,473
Granted | shares	183,177
Exercised/Converted | shares	(516,478)
Cancelled/Forfeited | shares	(89,926)
Ending Balance | shares	1,666,246
Weighted Average Exercise Price
Beginning Balance | $ / shares	$ 10.15
Granted | $ / shares	9.48
Exercised/Converted | $ / shares	7.89
Cancelled/Forfeited | $ / shares	10.65
Ending Balance | $ / shares	$ 10.75