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		6 Months Ended		36 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Target Performance Shares Outstanding
Beginning of period (in shares)			39,302	40,784	40,828	42,458
Shares paid (in shares)			0	0	(14,625)	(14,117)
New grants (in shares)			150	475	13,225	13,475	750
Forfeitures and cancellations (in shares)			(3)	(7)	(21)	(564)
Expense recognized (in shares)			0	0	0	0
End of period (in shares)	39,302	40,784	39,449	41,252	39,449	41,252	40,828
Accrued Expense
Beginning of period			$ 21.9	$ 38.2	$ 42.2	$ 56.3
Shares paid			0.0	0.0	(26.4)	(34.6)
New grants			0.0	0.0	0.0	0.0
Forfeitures and cancellations			(0.4)	(0.2)	(0.1)	0.2
Expense recognized			19.7	7.7	25.5	23.8
End of period	$ 21.9	$ 38.2	$ 41.2	$ 45.7	$ 41.2	$ 45.7	$ 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%