Employee benefit plans - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Shares (in thousands)
Outstanding at beginning of period (shares)	9,809
Granted (shares)	1,333
Exercised (shares)	(654)
Forfeited/Cancelled (shares)	(4,495)
Outstanding at end of period (shares)	5,993	9,809
Weighted- average exercise price
Outstanding at beginning of period (in dollars per share)	$ 11.16
Granted (usd per share)	5.77
Exercised (usd per share)	0.74
Outstanding at end of period (in dollars per share)	$ 7.28	$ 11.16
Weighted Average Remaining Contractual Term (in years)	5 years 5 months 7 days	6 years
Aggregate intrinsic value (in thousands)	$ 7,897	$ 19,971
Vested and Expected to Vest (shares)	5,988
Vested and Expected to Vest - Weighted Average Exercise Price (in dollars per share)	$ 7.28
Vested and Expected to Vest- Weighted Average Remaining Contractual Term	5 years 5 months 7 days
Vested and Expected to Vest - Aggregate Intrinsic Value	$ 7,897
Exercisable (shares)	4,459
Exercisable - Weighted average exercise price (in dollars per share)	$ 7.40
Exercisable - Weighted Average Remaining Contractual Term	4 years 2 months 12 days
Exercisable - Aggregate intrinsic value	$ 7,897
Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price	$ 16.26