Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	34,525
Assumed forfeitures (in shares)	(518)
Total target performance shares outstanding (in shares)	34,007	37,031	37,031	39,449
Accrued Expense	$ 45.0
Assumed forfeitures	(0.8)
Total	$ 44.2	$ 69.4	$ 36.4	$ 67.5
2022 – 2024
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,225
Accrued Expense	$ 35.0
2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,895
Accrued Expense	$ 9.4
2024 – 2026
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,405
Accrued Expense	$ 0.6