v3.10.0.1
Stock incentive plans - Stock option awards (Details) - Stock options - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Number of Options    
Outstanding at beginning of the period (in shares) 1,283  
Exercised (in shares) (871)  
Canceled/forfeited (in shares) (7)  
Outstanding and exercisable at end of the period (in shares) 405 1,283
Weighted Average Exercise Price    
Outstanding at beginning of the period (in dollars per share) $ 9.58  
Exercised (in dollars per share) 11.21  
Canceled/forfeited (in dollars per share) 5.99  
Outstanding and exercisable at end of the period (in dollars per share) $ 6.14 $ 9.58
Weighted-Average Remaining Contract Life (years)    
Outstanding 3 years 4 months 24 days 3 years 9 months 18 days
Aggregate Intrinsic Value    
Outstanding and exercisable at end of the period $ 11,660 $ 16,573
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%