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	6 Months Ended
	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	32,885
Assumed forfeitures (in shares)	(493)
Total target performance shares outstanding (in shares)	32,392	32,392	34,859	34,992	34,007	37,031
Accrued Expense	$ 30.3
Assumed forfeitures	(0.5)
Total	$ 29.8	$ 23.3	$ 71.1	$ 52.0	$ 44.2	$ 69.4
2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,835
Accrued Expense	$ 18.4
2024 – 2026
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	11,405
Accrued Expense	$ 9.6
2025 – 2027
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,645
Accrued Expense	$ 2.3