Short-Duration Insurance Contracts - Reconciliation of Net Incurred and Paid Claims (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
2024
Gross reserves, end of year	$ 569.2	$ 581.7
Property & Casualty and Group Benefits
2024
Liabilities for claims and claim adjustment expenses, net of reinsurance	388.3	390.4
Reinsurance recoverable, including reinsurance premium paid	128.7	138.9
Unallocated claims adjustment expenses	11.2	11.3
Total other than short duration and unallocated claims adjustment expenses	52.2	52.4
Gross reserves, end of year	569.2	581.7
Property & Casualty and Group Benefits \| Homeowners
2024
Liabilities for claims and claim adjustment expenses, net of reinsurance	63.3	73.4
Reinsurance recoverable (payable), including reinsurance premium paid	0.7	2.2
Property & Casualty and Group Benefits \| Automobile Liability
2024
Liabilities for claims and claim adjustment expenses, net of reinsurance	197.0	200.9
Reinsurance recoverable, including reinsurance premium paid	92.5	96.6
Property & Casualty and Group Benefits \| Auto physical damage
2024
Liabilities for claims and claim adjustment expenses, net of reinsurance	9.8	11.0
Property & Casualty and Group Benefits \| Specialty Health
2024
Liabilities for claims and claim adjustment expenses, net of reinsurance	7.3	10.9
Reinsurance recoverable, including reinsurance premium paid	0.1	0.2
Property & Casualty and Group Benefits \| Group Disability
2024
Liabilities for claims and claim adjustment expenses, net of reinsurance	77.9	83.2
Reinsurance recoverable, including reinsurance premium paid	22.6	25.0
Property & Casualty and Group Benefits \| Other than short duration lines
2024
Liabilities for claims and claim adjustment expenses, net of reinsurance	15.3	11.0
Reinsurance recoverable, including reinsurance premium paid	12.8	14.9
Property & Casualty and Group Benefits \| Legacy commercial exposures
2024
Liabilities for claims and claim adjustment expenses, net of reinsurance	17.7	0.0
Property & Casualty and Group Benefits \| Insurance lines other than short duration
2024
Insurance lines other than short duration	$ 41.0	$ 41.1

Liability For Other Than Short Duration And Unallocated Claims Adjustment Expenses

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- Definition

Reinsurance Recoverable (Payable) for Unpaid Claims and Claims Adjustments

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- Definition

The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

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- Definition

Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.

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Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

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Amount of reconciling item for the reconciliation of the liability in the disclosure of claims development (in excess of) less than the liability for unpaid claims and claims adjustment expense for short-duration insurance contracts, classified as other.

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Amount of liability for unallocated claim adjustment expense for short-duration insurance contracts.

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