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Investments Investments, Held in Trust or on Deposit (Details) - USD ($)
$ in Millions
Jun. 30, 2024
Dec. 31, 2023
Line of Credit Facility [Line Items]    
Other invested assets (cost $86.5 at June 30, 2024 and $82.5 at December 31, 2023) $ 97.5 $ 91.5
Deposit Assets [Abstract]    
Investments 2,376.1 2,278.1
Assets Held-in-trust [Abstract]    
Funds Held under Reinsurance Agreements, Asset 3.0 3.0
Federal Home Loan Bank [Member]    
Deposit Assets [Abstract]    
Line of Credit Facility, Maximum Borrowing Capacity 70.0 70.0
Required by various state laws and regulations to hold securities or letters of credit in depository account [Member]    
Deposit Assets [Abstract]    
Investments $ 758.6 $ 748.1