v3.8.0.1
Stock incentive plans - Restricted stock unit awards (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
RSUs  
Stock incentive plans  
Vesting period 3 years
Number of Shares  
Non-vested at beginning of year (in shares) 3,825,000
Granted (in shares) 617,000
Vested (in shares) (974,094)
Canceled/ forfeited (in shares) (144,000)
Non vested at end of year (in shares) 3,324,000
Weighted Average Grant Date Fair Value  
Non-vested at beginning of year (in dollars per share) $ 6.55
Granted (in dollars per share) 13.00
Vested (in dollars per share) 7.57
Canceled/forfeited (in dollars per share) 8.84
Non vested at end of year (in dollars per share) $ 7.35
Other Disclosures  
Shares of common stock issued resulting from vesting 656,767
Total unrecognized stock-based compensation expense $ 13,391
Weighted average period over which unrecognized compensation expense is expected to be recognized 2 years 9 months 18 days
Time-based restricted stock unit awards | Executive And Non Executive Member | Maximum  
Stock incentive plans  
Vesting period 3 years
Time-based restricted stock unit awards | Non-employee directors and existing members  
Stock incentive plans  
Vesting period 1 year
Time-based restricted stock unit awards | Non-employee directors and new members  
Stock incentive plans  
Vesting period 4 years
Vesting percentage when the individual completes 12 months of continuous service 25.00%
Performance-based restricted stock unit awards  
Stock incentive plans  
Vesting period 3 years