Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	37,595
Assumed forfeitures (in shares)	(564)
Total target performance shares outstanding (in shares)	37,031	37,031	39,449	39,449	39,449	40,828
Accrued Expense	$ 55.9
Assumed forfeitures	(0.8)
Total	$ 55.1	$ 44.6	$ 67.5	$ 53.0	$ 41.2	$ 42.2
2021 – 2023
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,475
Accrued Expense	$ 30.4
2022 – 2024
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,225
Accrued Expense	$ 22.1
2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,895
Accrued Expense	$ 3.4