Stock incentive plans - Stock option awards (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Number of Options
Outstanding at the beginning of the period (in shares)	3,084
Exercised (in shares)	(1,665)
Canceled/forfeited (in shares)	(20)
Outstanding at the end of the period (in shares)	1,399	3,084
Exercisable at the end of the period (in shares)	1,385
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 7.04
Exercised (in dollars per share)	4.80
Canceled/forfeited (in dollars per share)	6.69
Outstanding at the end of the period (in dollars per share)	9.70	$ 7.04
Exercisable at the end of the period (in dollars per share)	$ 9.74
Weighted-Average Remaining Contract Life (years)
Outstanding	4 years	3 years 9 months 18 days
Exercisable at the end of the period	4 years
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 16,318	$ 17,145
Exercisable at the end of the period	$ 16,105
Stock options
Stock incentive plans
Vesting period	4 years
Vesting percentage when the individual completes 12 months of continuous service	25.00%
Continuous service period of individual	12 months
Vesting percentage on a monthly basis	75.00%