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		9 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Target Performance Shares Outstanding
Beginning of period (in shares)			41,252	42,458	42,458	42,473
Shares paid (in shares)			(219)	0	(14,336)	(14,070)
New grants (in shares)			0	0	13,475	14,055
Forfeitures and cancellations (in shares)			(205)	0	(769)	0
Expense recognized (in shares)			0	0	0	0
End of period (in shares)			40,828	42,458	40,828	42,458
Accrued Expense
Beginning of period			$ 45.7	$ 15.8	$ 56.3	$ 43.7
Shares paid			(0.6)	0.0	(35.2)	(27.7)
New grants			0.0	0.0	0.0	0.0
Forfeitures and cancellations			0.2	(0.3)	0.4	0.1
Expense recognized			(6.5)	13.6	17.3	13.0
End of period			$ 38.8	$ 29.1	$ 38.8	$ 29.1
Performance Cycle 2018 to 2020
Accrued Expense
Percentage of target payout - minimum actual	200.00%
Performance Cycle 2017 to 2019
Accrued Expense
Percentage of target payout - minimum actual		174.00%
Percentage of target payout - maximum actual		180.00%