v3.6.0.2
Stock incentive plans - Stock option awards (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Aggregate Intrinsic Value      
Cash proceeds from exercise of stock option $ 2,984 $ 1,373 $ 1,158
Stock options      
Stock incentive plans      
Vesting period 4 years    
Number of Options      
Outstanding at the beginning of the year (in shares) 3,748,000    
Exercised (in shares) (532,000) (440,000) (458,000)
Canceled/forfeited (in shares) (132,000)    
Outstanding at the end of the year (in shares) 3,084,000 3,748,000  
Exercisable at the end of the year (in shares) 2,971,000    
Weighted Average Exercise Price      
Outstanding at the beginning of the year (in dollars per share) $ 6.89    
Exercised (in dollars per share) 5.61    
Canceled/forfeited (in dollars per share) 8.79    
Outstanding at the end of the year (in dollars per share) 7.04 $ 6.89  
Exercisable at the end of the year (in dollars per share) $ 7.07    
Weighted-Average Remaining Contract Life (years)      
Outstanding 3 years 9 months 18 days 5 years  
Exercisable at the end of the year 3 years 7 months 6 days    
Aggregate Intrinsic Value      
Outstanding $ 17,145 $ 6,611  
Exercisable at the end of the year 16,468    
Cash proceeds from exercise of stock option 2,984 1,373 $ 1,158
Total intrinsic value of stock options exercised (in dollars) 1,675 $ 2,214 $ 2,027
Weighted average grant-date fair value of stock options granted (in dollars per share)     $ 2.92
Other disclosures      
Total unrecognized stock-based compensation expense $ 323    
Weighted average period over which unrecognized compensation expense is expected to be recognized 9 months 18 days    
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%