Employee Share-Based Incentive Compensation Plans - Performance Shares (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		36 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2021
Performance Share
Target Performance Shares Outstanding
End of period (in shares)	37,031		37,031
Accrued Expense
End of period	$ 36.4		$ 36.4
Performance Share | W T M Incentive Plan
Target Performance Shares Outstanding
Beginning of period (in shares)			39,449	40,828
Shares paid (in shares)			(13,350)	(14,625)
New grants (in shares)			10,895	13,075	750
Forfeitures and cancellations (in shares)			37	24
Expense recognized (in shares)			0	0
End of period (in shares)					40,828
Accrued Expense
Beginning of period			$ 67.5	$ 42.2
Shares paid			(40.8)	(26.4)
New grants			0.0	0.0
Forfeitures and cancellations			0.4	0.3
Expense recognized			$ 9.3	$ 5.8
End of period					$ 42.2
Performance Share | W T M Incentive Plan | Range Of Performance Cycle Current Year
Accrued Expense
Percentage of target payout - minimum actual	200.00%
Performance Share | W T M Incentive Plan | Range Of Performance Cycle Previous Year
Accrued Expense
Percentage of target payout - minimum actual		172.00%
W T M Incentive Plan
Target Performance Shares Outstanding
End of period (in shares)	37,031	39,302	37,031	39,302
Accrued Expense
End of period	$ 36.4	$ 21.9	$ 36.4	$ 21.9