Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - Performance Share - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	37,595
Total target performance shares outstanding (in shares)	37,031
Accrued Expense	$ 37.0
Total Accrued Expense	$ 36.4
W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Assumed forfeitures (in shares)	(564)
Total target performance shares outstanding (in shares)		39,449	40,828
Assumed forfeitures	$ (0.6)
Total Accrued Expense		$ 67.5	$ 42.2
W T M Incentive Plan | 2021 – 2023
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,475
Accrued Expense	$ 22.6
W T M Incentive Plan | 2022 – 2024
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	13,225
Accrued Expense	$ 14.0
W T M Incentive Plan | 2023 – 2025
Share-based compensation arrangement by share-based payment award
Target performance shares outstanding (in shares)	10,895
Accrued Expense	$ 0.4