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	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	40,050
Assumed forfeitures (in shares)	(601)
Total target performance shares outstanding (in shares)	39,449	39,302	40,828	41,252	40,784	42,458
Accrued Expense	$ 41.9
Assumed forfeitures	(0.7)
Total Accrued Expense	$ 41.2	$ 21.9	$ 42.2	$ 45.7	$ 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	$ 27.4
2021 – 2023
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,475
Accrued Expense	$ 11.0
2022 – 2024
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,225
Accrued Expense	$ 3.5