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Share-Based Compensation - Restricted Stock Activity (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 9 Months Ended 120 Months Ended
Jan. 31, 2021
Sep. 30, 2021
Dec. 31, 2019
Dec. 31, 2020
Time-Based Restricted Stock Units        
Number of Units        
Beginning balance (in shares) 563,224 563,224    
Granted (in shares)   218,985    
Market-performance adjustment (in shares)   0    
Vested (in shares)   (221,936) (615,643)  
Forfeited (in shares)   (64,415)    
Ending balance (in shares)   495,858    
Nonvested shares, aggregate intrinsic value   $ 13.8   $ 21.0
Shares vested, but not released (in shares)   615,643    
Shares vested, but not released, aggregate intrinsic value   $ 17.9    
Total outstanding awards (in shares)   1,111,501    
Total outstanding awards, aggregate intrinsic value   $ 31.7    
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    
Nonvested shares, aggregate intrinsic value   $ 0.8   1.6
Shares vested, but not released (in shares)   42,858    
Shares vested, but not released, aggregate intrinsic value   $ 1.2    
Total outstanding awards (in shares)   71,428    
Total outstanding awards, aggregate intrinsic value   $ 2.0    
Market-Based Restricted Stock Units        
Number of Units        
Beginning balance (in shares) 873,581 873,581    
Granted (in shares)   108,781    
Market-performance adjustment (in shares)   266,531    
Vested (in shares) (799,591) (800,087)    
Forfeited (in shares)   (69,716)    
Ending balance (in shares)   379,090    
Nonvested shares, aggregate intrinsic value   $ 10.5   $ 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)   379,090    
Total outstanding awards, aggregate intrinsic value   $ 10.5    
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