Stock-based compensation - Stock Options and RSUs (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee stock options
Shares Available for Grant
Beginning Balance (in shares)	959,136
Authorized for issuance (in shares)	1,947,667
Granted (in shares)	(2,300,056)
Exercised/Converted (in shares)	0
Cancelled/Forfeited (in shares)	(495,576)
Ending Balance (in shares)	1,102,323	959,136
Number of Shares
Beginning Balance (in shares)	3,933,529
Authorized for issuance (in shares)	0
Granted (in shares)	158,116
Exercised/Converted (in shares)	(778,811)
Cancelled/Forfeited (in shares)	(110,089)
Ending Balance (in shares)	3,202,745	3,933,529
Weighted Average Exercise Price
Beginning Balance (USD per share)	$ 5.55
Authorized for issuance (in shares)	0.00
Granted (USD per share)	6.64
Exercised/Converted (USD per share)	4.39
Cancelled/Forfeited (USD per share)	10.07
Ending Balance (USD per share)	$ 5.73	$ 5.55
Restricted stock units
Number of Units
Beginning Balance (in shares)	2,404,637
Authorized for issuance (in shares)	0
Granted (in shares)	1,446,940
Exercised/Converted (in shares)	(1,123,893)
Cancelled/Forfeited (in shares)	(241,656)
Ending Balance (in shares)	2,486,028	2,404,637
Weighted Average Exercise Price
Beginning Balance (USD per share)	$ 9.02
Authorized for issuance (in shares)	0.00
Granted (USD per share)	6.83	$ 7.86	$ 12.33
Exercised/Converted (USD per share)	8.82
Cancelled/Forfeited (USD per share)	8.74
Ending Balance (USD per share)	$ 7.87	$ 9.02