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Fair Value of Financial Instruments - Narrative (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2024
USD ($)
security
Dec. 31, 2023
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Percentage of investment portfolio fair value pricing services or index price 90.90% 87.70%
Net investment gains (losses) included in net income $ 0.9 $ 0.4
Net investment (gains) losses included in net income related to financial liabilities $ 5.4 1.3
Fair Value Recurring Basis Unobservable Input Reconciliation Liability Gain Loss Statement Of Income Extensible List Not Disclosed Flag true  
Equity securities at fair value, (cost $78.8 and $86.2) $ 66.5 86.2
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) 1,121.3 1,138.8
Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) 28.9 29.5
Carrying Value | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities at fair value, (cost $78.8 and $86.2) 66.5 86.2
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) 28.9  
Fair Value | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities at fair value, (cost $78.8 and $86.2) 66.5 86.2
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) $ 28.9  
Fair Value, Inputs, Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Percentage of invested assets in total investment portfolio Level 3 recurring 9.50%  
Fair Value, Inputs, Level 3 | Fair Value | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities at fair value, (cost $78.8 and $86.2) $ 4.2 4.5
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) 28.9  
Fair Value, Inputs, Level 3 | Fair Value | Fair Value, Recurring | External    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities at fair value, (cost $78.8 and $86.2) 4.2 4.5
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) 28.9  
Fair Value, Inputs, Level 1 [Member] | Fair Value | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities at fair value, (cost $78.8 and $86.2) 1.4 17.9
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) 0.0  
Fair Value, Inputs, Level 2 [Member] | Fair Value | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities at fair value, (cost $78.8 and $86.2) 60.9 $ 63.8
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) $ 0.0  
Minimum    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Number of hard-to-value fixed maturity securities | security 75  
Maximum    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Number of hard-to-value fixed maturity securities | security 125