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, 2022	Jun. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 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	39,449	40,828	40,828	41,252	42,458
Accrued Expense	$ 53.8
Assumed forfeitures	(0.8)
Total Accrued Expense	$ 53.0	$ 41.2	$ 42.2	$ 38.8	$ 45.7	$ 56.3
2020 – 2022
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,350
Accrued Expense	$ 31.9
2021 – 2023
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,475
Accrued Expense	$ 14.7
2022 – 2024
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,225
Accrued Expense	$ 7.2