Employee Share-Based Incentive Compensation Plans - Performance Shares (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Target Performance Shares Outstanding
Beginning of period (in shares)	42,458	42,473	42,473	40,616
Shares paid (in shares)	0	0	(14,070)	(13,715)
New grants (in shares)	0	0	14,055	15,600
Forfeitures and cancellations (in shares)	0	0	0	(28)
End of period (in shares)	42,458	42,473	42,458	42,473
Accrued Expense
Beginning of period	$ 15.8	$ 30.8	$ 43.7	$ 31.7
Shares paid	0.0	0.0	(27.7)	(18.1)
Forfeitures and cancellations	(0.3)	0.0	0.1	0.0
Expense recognized	13.6	5.6	13.0	22.8
End of period	$ 29.1	$ 36.4	$ 29.1	$ 36.4
Performance Cycle 2018 to 2020
Accrued Expense
Percentage of target payout - minimum actual			174.00%
Percentage of target payout - maximum actual			180.00%
Performance Cycle 2016 to 2018
Accrued Expense
Percentage of target payout - minimum actual				139.00%
Percentage of target payout - maximum actual				166.00%