Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2026	Jun. 30, 2026	Dec. 31, 2025	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	31,665
Accrued Expense	$ 50.0
Assumed forfeitures (in shares)	(475)
Assumed forfeitures	$ (0.8)
Total target performance shares outstanding (in shares)	31,190	31,190	32,416	32,416	32,392	34,859
Total	$ 49.2	$ 48.0	$ 73.2	$ 52.0	$ 29.8	$ 71.1
2024 – 2026
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	11,405
Accrued Expense	$ 29.3
2025 – 2027
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,670
Accrued Expense	$ 16.7
2026 – 2028
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	9,590
Accrued Expense	$ 4.0