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, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Restricted Shares
Beginning of period (in shares)	37,595	38,200	38,350	37,850
Issued (in shares)	0	150	10,895	13,225
Vested (in shares)	0	0	(11,650)	(12,725)
Forfeited (in shares)	0	0	0	0
Expense recognized (in shares)	0	0	0	0
End of period (in shares)	37,595	38,350	37,595	38,350
Unamortized Issue Date Fair Value
Beginning of period	$ 28.3	$ 26.5	$ 15.5	$ 15.9
Issued	0.0	0.2	16.0	13.8
Vested	0.0	0.0	0.0	0.0
Forfeited	0.0	0.0	0.0	0.0
Expense recognized	(4.0)	(3.7)	(7.2)	(6.7)
End of period	$ 24.3	$ 23.0	$ 24.3	$ 23.0