Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	32,910
Accrued Expense	$ 74.3
Assumed forfeitures (in shares)	(494)
Assumed forfeitures	$ (1.1)
Total target performance shares outstanding (in shares)	32,416	34,859	37,031	39,449
Total	$ 73.2	$ 71.1	$ 69.4	$ 67.5
2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,835
Accrued Expense	$ 41.3
2024 – 2026
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	11,405
Accrued Expense	$ 23.0
2025 – 2027
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,670
Accrued Expense	$ 10.0