Stock incentive plans - Stock option awards (Details) - Stock options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Number of Options
Outstanding at beginning of the period (in shares)	1,283
Exercised (in shares)	(735)
Canceled/forfeited (in shares)	(7)
Outstanding at end of the period (in shares)	541	1,283
Exercisable at end of the period (in shares)	541
Weighted Average Exercise Price
Outstanding at beginning of the period (in dollars per share)	$ 9.58
Exercised (in dollars per share)	11.50
Canceled/forfeited (in dollars per share)	5.99
Outstanding at end of the period (in dollars per share)	7.02	$ 9.58
Exercisable at end of the period (in dollars per share)	$ 7.02
Weighted-Average Remaining Contract Life (years)
Outstanding	3 years 7 months 6 days	3 years 9 months 18 days
Exercisable at end of the period	3 years 7 months 6 days
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 8,426	$ 16,573
Exercisable at end of the period	$ 8,426
Vesting in 12 months
Stock incentive plans
Vesting period	12 months
Vesting percentage	25.00%
Vesting monthly 12 months after grant date
Stock incentive plans
Vesting period	4 years
Vesting percentage	75.00%