Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	32,910
Assumed forfeitures (in shares)	(494)
Total target performance shares outstanding (in shares)	32,416	32,392	34,859	34,859	34,992	37,031
Accrued Expense	$ 52.8
Assumed forfeitures	(0.8)
Total	$ 52.0	$ 29.8	$ 71.1	$ 55.7	$ 52.0	$ 69.4
2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,835
Accrued Expense	$ 30.6
2024 – 2026
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	11,405
Accrued Expense	$ 16.5
2025 – 2027
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,670
Accrued Expense	$ 5.7