Employee Share-Based Incentive Compensation Plans - Restricted Shares (Details) - W T M Incentive Plan - Restricted Stock - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Restricted Shares
Beginning of period (in shares)	38,200	37,805	37,850	43,105
Issued (in shares)	150	475	13,225	13,475
Vested (in shares)	0	0	(12,725)	(17,717)
Forfeited (in shares)	0	0	0	(583)
Expense recognized (in shares)	0	0	0	0
End of period (in shares)	38,350	38,280	38,350	38,280
Unamortized Issue Date Fair Value
Beginning of period	$ 26.5	$ 26.2	$ 15.9	$ 15.2
Issued	0.2	0.5	13.8	16.1
Vested	0.0	0.0	0.0	0.0
Forfeited	0.0	0.0	0.0	(0.6)
Expense recognized	(3.7)	(3.5)	(6.7)	(7.5)
End of period	$ 23.0	$ 23.2	$ 23.0	$ 23.2