Unpaid Claims and Claim Expense Reserves - Reconciliation of Net Incurred and Paid Claims (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Gross reserves, end of year	$ 585.1	$ 425.9
Property & Casualty and Group Benefits
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	360.5
Reinsurance recoverables, including reinsurance premium paid	139.0
Unallocated claims adjustment expenses	21.0
Total other than short duration and unallocated claims adjustment expenses	85.6
Gross reserves, end of year	585.1
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	61.0
Reinsurance recoverables, including reinsurance premium paid	(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	192.5
Reinsurance recoverables, including reinsurance premium paid	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	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	16.9
Reinsurance recoverables, including reinsurance premium paid	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	69.0
Reinsurance recoverables, including reinsurance premium paid	25.6
Property & Casualty and Group Benefits | Other short duration lines
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	10.5
Reinsurance recoverables, including reinsurance premium paid	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	$ 64.6