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Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - W T M Incentive Plan - Restricted Stock - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Restricted Shares    
Beginning of period (in shares) 43,105 43,395
Issued (in shares) 13,000 14,055
Vested (in shares) (17,717) (14,345)
Forfeited (in shares) (583) 0
Expense recognized (in shares) 0 0
End of period (in shares) 37,805 43,105
Unamortized Issue Date Fair Value    
Beginning of period $ 15.2 $ 16.7
Issued 15.6 15.1
Vested 0.0 0.0
Forfeited (0.6) 0.0
Expense recognized (4.0) (5.2)
End of period $ 26.2 $ 26.6