Short-Duration Insurance Contracts - Reconciliation of Net Incurred and Paid Claims (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	$ 586.6		$ 564.0
Total short-duration insurance contracts
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	536.4		510.3
Property & Casualty
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	419.3		388.7
Group Benefits
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	117.1	$ 121.9	121.6	$ 123.8	$ 130.2	$ 125.4
Other than short-duration
Short-Duration Insurance Contracts, Historical Claims Duration [Line Items]
Unpaid claims and claim expenses	$ 50.2		$ 53.7