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Stock incentive plans - Restricted stock units (Details)
9 Months Ended
Sep. 30, 2017
$ / shares
shares
RSUs  
Stock incentive plans  
Shares of common stock issued resulting from vesting 560,661
Number of Shares  
Non-vested at beginning of period (in shares) 3,825,000
Granted (in shares) 598,000
Vested (in shares) (814,583)
Canceled/ forfeited (in shares) (139,000)
Non-vested at end of period (in shares) 3,469,000
Weighted Average Grant Date Fair Value  
Non-vested at beginning period (in dollars per share) | $ / shares $ 6.55
Granted (in dollars per share) | $ / shares 12.69
Vested (in dollars per share) | $ / shares 7.36
Canceled/forfeited (in dollars per share) | $ / shares 8.85
Non-vested at end of period (in dollars per share) | $ / shares $ 7.33
Time-based restricted stock unit awards | Executive And Non Executive Member  
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 per year (as a percent) 25.00%
Vesting period 4 years
Performance-based restricted stock unit awards | Executive members  
Stock incentive plans  
Vesting period 3 years