Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	37,595
Accrued Expense	$ 70.5
Assumed forfeitures (in shares)	(564)
Assumed forfeitures	$ (1.1)
Total target performance shares outstanding (in shares)	37,031	39,449	40,828	42,458
Total	$ 69.4	$ 67.5	$ 42.2	$ 56.3
2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,895
Accrued Expense	$ 6.0
2022 – 2024
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,225
Accrued Expense	$ 25.8
2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,475
Accrued Expense	$ 38.7