v3.8.0.1
Stock incentive plans - Stock option awards (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Aggregate Intrinsic Value      
Cash proceeds from exercise of stock option $ 9,244 $ 2,984 $ 1,373
Stock options      
Stock incentive plans      
Vesting period 4 years    
Number of Options      
Outstanding at beginning of year (in shares) 3,084,000    
Exercised (in shares) (1,776,000) (532,000) (440,000)
Canceled/forfeited (in shares) (25,000)    
Outstanding at end of year (in shares) 1,283,000 3,084,000  
Exercisable at end of year (in shares) 1,282,000    
Weighted Average Exercise Price      
Outstanding at beginning of year (in dollars per share) $ 7.04    
Exercised (in dollars per share) 5.21    
Canceled/forfeited (in dollars per share) 6.56    
Outstanding at end of year (in dollars per share) 9.58 $ 7.04  
Exercisable at end of year (in dollars per share) $ 9.58    
Weighted-Average Remaining Contract Life (years)      
Outstanding 3 years 9 months 18 days 3 years 9 months 18 days  
Exercisable at end of year 3 years 9 months 18 days    
Aggregate Intrinsic Value      
Outstanding $ 16,573 $ 17,145  
Exercisable at end of year 16,560    
Total intrinsic value of stock options exercised (in dollars) $ 20,551 $ 1,675 $ 2,214
Stock options | Vesting in 12 months      
Stock incentive plans      
Vesting period 12 months    
Vesting percentage 25.00%    
Stock options | Vesting monthly 12 months after grant date      
Stock incentive plans      
Vesting period 4 years    
Vesting percentage 75.00%