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Employee Share-Based Incentive Compensation Plans - Performance Shares (Details) - Performance Share - W T M Incentive Plan - USD ($)
$ in Millions
 1 Months Ended 3 Months Ended 6 Months Ended 36 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Jun. 30, 2023
Jun. 30, 2022
Jun. 30, 2023
Jun. 30, 2022
Dec. 31, 2021
Target Performance Shares Outstanding              
Beginning of period (in shares)     37,031 39,302 39,449 40,828  
Shares paid (in shares)     0 0 (13,350) (14,625)  
New grants (in shares)     0 150 10,895 13,225 750
Forfeitures and cancellations (in shares)     0 (3) 37 21  
Expense recognized (in shares)     0 0 0 0  
End of period (in shares) 37,031 39,302 37,031 39,449 37,031 39,449 40,828
Accrued Expense              
Beginning of period     $ 36.4 $ 21.9 $ 67.5 $ 42.2  
Shares paid     0.0 0.0 (40.8) (26.4)  
New grants     0.0 0.0 0.0 0.0  
Forfeitures and cancellations     (0.2) (0.4) 0.2 (0.1)  
Expense recognized     8.4 19.7 17.7 25.5  
End of period $ 36.4 $ 21.9 $ 44.6 $ 41.2 $ 44.6 $ 41.2 $ 42.2
Range Of Performance Cycle Current Year              
Accrued Expense              
Percentage of target payout - minimum actual 200.00%            
Range Of Performance Cycle Previous Year              
Accrued Expense              
Percentage of target payout - minimum actual   172.00%