Employee benefit plans - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Shares (in thousands)
Outstanding at beginning of period (shares)	13,081
Granted (shares)	2,573
Exercised (shares)	(1,733)
Forfeited/Cancelled (shares)	(1,542)
Outstanding at end of period (shares)	12,379	13,081
Weighted- average exercise price
Outstanding at beginning of period (in dollars per share)	$ 11.82
Granted (usd per share)	11.27
Exercised (usd per share)	2.05
Forfeited/Cancelled (usd per share)	19.07
Outstanding at end of period (in dollars per share)	$ 12.17	$ 11.82
Weighted- average remaining contractual term (in years)	5 years 11 months 19 days	6 years 8 months 12 days
Aggregate intrinsic value (in thousands)	$ 32,772	$ 108,846
Vested and expected to vest (shares)	12,245
Vested and expected to vest - Weighted average exercise price (in dollars per share)	$ 12.12
Vested and expected to vest - Weighted average remaining contractual term	5 years 11 months 12 days
Vested and expected to vest - Aggregate intrinsic value	$ 32,772
Exercisable (shares)	8,952
Exercisable - Weighted average exercise price (in dollars per share)	$ 10.37
Exercisable - Weighted-average remaining contractual term	5 years 4 months 10 days
Exercisable - Aggregate intrinsic value	$ 32,771