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	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	39,900
Assumed forfeitures (in shares)	(598)
Total target performance shares outstanding (in shares)	39,302	40,828	40,784	42,458
Accrued Expense	$ 22.2
Assumed forfeitures	(0.3)
Total Accrued Expense	$ 21.9	$ 42.2	$ 38.2	$ 56.3
2020 – 2022
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,350
Accrued Expense	$ 19.0
2021 – 2023
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,475
Accrued Expense	$ 2.9
2022 – 2024
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,075
Accrued Expense	$ 0.3