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, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Restricted Shares
Beginning of period (in shares)	38,350	38,280	37,850	43,105
Issued (in shares)	0	0	13,225	13,475
Vested (in shares)	0	(219)	(12,725)	(17,936)
Forfeited (in shares)	0	(211)	0	(794)
Expense recognized (in shares)	0	0	0	0
End of period (in shares)	38,350	37,850	38,350	37,850
Unamortized Issue Date Fair Value
Beginning of period	$ 23.0	$ 23.2	$ 15.9	$ 15.2
Issued	0.0	0.0	13.8	16.1
Vested	0.0	0.0	0.0	0.0
Forfeited	0.0	(0.2)	0.0	(0.8)
Expense recognized	(3.8)	(3.6)	(10.5)	(11.1)
End of period	$ 19.2	$ 19.4	$ 19.2	$ 19.4