Short-Duration Insurance Contracts - Reconciliation of Net Incurred and Paid Claims (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Gross reserves, end of year	$ 581.7	$ 564.0	[1]
Property & Casualty and Group Benefits
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	390.4	360.5
Reinsurance recoverable, including reinsurance premium paid	138.9	139.0
Unallocated claims adjustment expenses	11.3	21.0
Total other than short duration and unallocated claims adjustment expenses	52.4	64.5
Gross reserves, end of year	581.7	564.0
Property & Casualty and Group Benefits | Homeowners
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	73.4	61.0
Reinsurance recoverable (payable), including reinsurance premium paid	2.2	(4.4)
Property & Casualty and Group Benefits | Auto liability
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	200.9	192.5
Reinsurance recoverable, including reinsurance premium paid	96.6	97.6
Property & Casualty and Group Benefits | Auto physical damage
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	11.0	10.6
Property & Casualty and Group Benefits | Specialty health
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	10.9	16.9
Reinsurance recoverable, including reinsurance premium paid	0.2	0.2
Property & Casualty and Group Benefits | Group disability
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	83.2	69.0
Reinsurance recoverable, including reinsurance premium paid	25.0	25.6
Property & Casualty and Group Benefits | Other than short duration lines
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	11.0	10.5
Reinsurance recoverable, including reinsurance premium paid	14.9	20.0
Property & Casualty and Group Benefits | Insurance lines other than short duration
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Insurance lines other than short duration	$ 41.1	$ 43.5

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements