Short-Duration Insurance Contracts - Reconciliation of Net Incurred and Paid Claims (Details) - USD ($) $ in Millions	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	$ 597.8		$ 564.0
Total short-duration insurance contracts
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	537.7		510.3
Property & Casualty
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	415.8		388.7
Group Benefits
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	121.9	$ 120.3	121.6	$ 130.2	$ 133.2	$ 125.4
Other than short-duration
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	$ 60.1		$ 53.7