v3.8.0.1
Stock incentive plans - Restricted stock unit awards (Details) - $ / shares
1 Months Ended 3 Months Ended
Feb. 28, 2018
Mar. 31, 2018
RSUs    
Stock incentive plans    
Shares of common stock issued resulting from vesting   362,000
Number of Shares    
Non-vested at beginning of period (in shares)   3,324,000
Granted (in shares)   874,000
Vested (in shares)   (609,000)
Canceled/forfeited (in shares)   (33,000)
Non vested at end of period (in shares)   3,556,000
Weighted Average Grant-Date Fair Value    
Non-vested at beginning of period (in dollars per share)   $ 7.35
Granted (in dollars per share)   12.18
Vested (in dollars per share)   7.20
Canceled/forfeited (in dollars per share)   12.55
Non vested at end of period (in dollars per share)   $ 8.51
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 percentage when the individual completes 12 months of continuous service   25.00%
Vesting period   4 years
Performance-based restricted stock unit awards    
Stock incentive plans    
Vesting period   3 years
Number of Shares    
Granted (in shares) 520,000  
Weighted Average Grant-Date Fair Value    
Granted (in dollars per share) $ 6.13  
Performance-based restricted stock unit awards | Minimum    
Stock incentive plans    
Performance objective threshold (as a percent)   50.00%
Performance-based restricted stock unit awards | Maximum    
Stock incentive plans    
Performance objective threshold (as a percent)   150.00%