Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	41,450
Assumed forfeitures (in shares)	(622)
Total target performance shares outstanding (in shares)	40,828	41,252	42,458	42,458	42,458	42,473
Accrued Expense	$ 39.4
Assumed forfeitures	(0.6)
Total Accrued Expense	$ 38.8	$ 45.7	$ 56.3	$ 29.1	$ 15.8	$ 43.7
2019 – 2021
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	14,625
Accrued Expense	$ 24.4
2020 – 2022
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,350
Accrued Expense	$ 13.4
2021 – 2023
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,475
Accrued Expense	$ 1.6