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Share-Based Compensation - Restricted Stock Activity (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended 12 Months Ended 120 Months Ended
Jan. 31, 2021
Mar. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Time-Based Restricted Stock Units        
Number of Units        
Beginning balance (in shares) 563,224 563,224    
Granted (in shares)   188,088    
Market-performance adjustment (in shares)   0    
Vested (in shares)   (189,293)   (615,643)
Forfeited (in shares)   (11,526)    
Ending balance (in shares)   550,493 563,224  
Nonvested shares, aggregate intrinsic value   $ 21.2 $ 21.0  
Shares vested, but not released (in shares)   615,643    
Shares vested, but not released, aggregate intrinsic value   $ 23.7    
Total outstanding awards (in shares)   1,166,136    
Total outstanding awards, aggregate intrinsic value   $ 44.9    
Performance-Based Restricted Stock Units        
Number of Units        
Beginning balance (in shares) 42,856 42,856    
Granted (in shares)   0    
Market-performance adjustment (in shares)   0    
Vested (in shares)   (14,286)   (28,570)
Forfeited (in shares)   0    
Ending balance (in shares)   28,570 42,856  
Nonvested shares, aggregate intrinsic value   $ 1.1 $ 1.6  
Shares vested, but not released (in shares)   42,858    
Shares vested, but not released, aggregate intrinsic value   $ 1.6    
Total outstanding awards (in shares)   71,428    
Total outstanding awards, aggregate intrinsic value   $ 2.7    
Market-Based Restricted Stock Units        
Number of Units        
Beginning balance (in shares) 873,581 873,581    
Granted (in shares)   108,522    
Market-performance adjustment (in shares)   266,531    
Vested (in shares) (799,591) (799,591)    
Forfeited (in shares)   (13,076)    
Ending balance (in shares)   435,967 873,581  
Nonvested shares, aggregate intrinsic value   $ 16.8 $ 42.6  
Shares vested, but not released (in shares)   0    
Shares vested, but not released, aggregate intrinsic value   $ 0.0    
Total outstanding awards (in shares)   435,967    
Total outstanding awards, aggregate intrinsic value   $ 16.8    
Resigned Trustee | Restricted Stock Units        
Number of Units        
Vested (in shares)     (14,286)  
Shares vested, but not released (in shares)   568,753    
Weighted average grant date fair value of vested awards (in USD per share)   $ 9.38