Employee Share-Based Incentive Compensation Plans - Performance Shares (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	1 Months Ended		3 Months Ended		36 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Target Performance Shares Outstanding
Beginning of period (in shares)			40,828	42,458
Shares paid (in shares)			(14,625)	(14,117)
New grants (in shares)			13,075	13,000	750
Forfeitures and cancellations (in shares)			24	(557)
Expense recognized (in shares)			0	0
End of period (in shares)	39,302	40,784	39,302	40,784	40,828
Accrued Expense
Beginning of period			$ 42.2	$ 56.3
Shares paid			(26.4)	(34.6)
New grants			0.0	0.0
Forfeitures and cancellations			0.3	0.4
Expense recognized			5.8	16.1
End of period	$ 21.9	$ 38.2	$ 21.9	$ 38.2	$ 42.2
Range Of Performance Cycle Current Year
Accrued Expense
Percentage of target payout - minimum actual	172.00%
Range Of Performance Cycle Previous Year
Accrued Expense
Percentage of target payout - minimum actual		200.00%