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Share-Based Compensation - Restricted Stock Activity (Details)
$ / shares in Units, $ in Millions
9 Months Ended
Sep. 30, 2019
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
Time-Based Restricted Stock Units    
Number of Units    
Beginning balance (in shares) 1,028,256  
Granted (in shares) 276,134  
Vested (in shares) (377,257)  
Forfeited (in shares) (150,795)  
Ending balance (in shares) 776,338  
Aggregate intrinsic value | $ $ 28.8 $ 26.3
Performance-Based Restricted Stock Units    
Number of Units    
Beginning balance (in shares) 71,428  
Granted (in shares) 0  
Vested (in shares) (14,286)  
Forfeited (in shares) 0  
Ending balance (in shares) 57,142  
Aggregate intrinsic value | $ $ 2.1 1.8
Market-Based Restricted Stock Units    
Number of Units    
Beginning balance (in shares) 587,500  
Granted (in shares) 243,168  
Vested (in shares) 0  
Forfeited (in shares) (51,480)  
Ending balance (in shares) 779,188  
Aggregate intrinsic value | $ $ 28.9 $ 15.0
Restricted Stock Units    
Number of Units    
Vested awards (in shares) 627,890  
Vested awards, aggregate intrinsic value | $ $ 23.3  
Vested awards, aggregate intrinsic value (in USD per share) | $ / shares $ 37.07  
Resigned Trustee | Restricted Stock Units    
Number of Units    
Vested awards (in shares) 568,753