Short-Duration Insurance Contracts - Reconciliation of Net Incurred and Paid Claims (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	$ 584.4	$ 581.7
Total short-duration insurance contracts
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	532.5	533.4
Property & Casualty
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	418.2	416.8
Group Benefits
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	114.3	116.6	$ 120.3	$ 121.6
Other than short-duration
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	$ 51.9	$ 48.3