Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - W T M Incentive Plan - Restricted Stock - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Restricted Shares
Beginning of period (in shares)	38,280	43,105	43,105	43,395
Issued (in shares)	0	0	13,475	14,055
Vested (in shares)	(219)	0	(17,936)	(14,345)
Forfeited (in shares)	(211)	0	(794)	0
Expense recognized (in shares)	0	0	0	0
End of period (in shares)	37,850	43,105	37,850	43,105
Unamortized Issue Date Fair Value
Beginning of period	$ 23.2	$ 23.0	$ 15.2	$ 16.7
Issued	0.0	0.0	16.1	15.1
Vested	0.0	0.0	0.0	0.0
Forfeited	(0.2)	0.0	(0.8)	0.0
Expense recognized	(3.6)	(3.6)	(11.1)	(12.4)
End of period	$ 19.4	$ 19.4	$ 19.4	$ 19.4