Unpaid Claims and Claim Expenses - Reconciliation of Net Incurred and Paid Claims (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Short-duration Insurance Contracts, Historical Claims Duration
Gross reserves, end of year	$ 425.9	$ 438.8
Property & Casualty segment
Short-duration Insurance Contracts, Historical Claims Duration
Liabilities for claims and claim adjustment expenses, net of reinsurance	233.8
Reinsurance recoverables, including reinsurance premium paid	110.3
Unallocated claims adjustment expenses	18.3
Total other than short duration and unallocated claims adjustment expenses	81.8
Property & Casualty segment | Homeowners
Short-duration Insurance Contracts, Historical Claims Duration
Liabilities for claims and claim adjustment expenses, net of reinsurance	52.8
Reinsurance recoverables, including reinsurance premium paid	1.4
Property & Casualty segment | Auto liability
Short-duration Insurance Contracts, Historical Claims Duration
Liabilities for claims and claim adjustment expenses, net of reinsurance	171.5
Reinsurance recoverables, including reinsurance premium paid	99.9
Property & Casualty segment | Auto physical damage
Short-duration Insurance Contracts, Historical Claims Duration
Liabilities for claims and claim adjustment expenses, net of reinsurance	6.7
Property & Casualty segment | Other short duration lines
Short-duration Insurance Contracts, Historical Claims Duration
Liabilities for claims and claim adjustment expenses, net of reinsurance	2.8
Reinsurance recoverables, including reinsurance premium paid	9.0
Property & Casualty segment | Insurance lines other than short duration
Short-duration Insurance Contracts, Historical Claims Duration
Insurance lines other than short duration	$ 63.5