v3.25.1
Quantitative Information Regarding Significant Unobservable Inputs (Details)
$ in Millions
Mar. 31, 2025
USD ($)
Dec. 31, 2024
USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Debt Securities, Available-for-Sale $ 35,751.4 $ 35,629.9
Equity Securities 26.9 24.5
All Other Corporate Bonds    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Debt Securities, Available-for-Sale 24,893.9 24,822.2
Mortgage/Asset-backed Securities    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Debt Securities, Available-for-Sale 960.8 917.2
Fair Value, Inputs, Level 3 | Discounted Cash Flow | Embedded Derivative in Modified Coinsurance Arrangement    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Embedded Derivative, Fair Value of Embedded Derivative Asset [1] $ 9.6 $ 11.5
Fair Value, Inputs, Level 3 | Discounted Cash Flow | Weighted Average | Input Measurement, Spread of Swap Curve | Embedded Derivative in Modified Coinsurance Arrangement    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Embedded Derivative Liability, Measurement Input (0.0015) (0.0023)
Fair Value, Inputs, Level 3 | All Other Corporate Bonds | Market Approach    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Debt Securities, Available-for-Sale [2],[3] $ 15.7 $ 16.3
Fair Value, Inputs, Level 3 | All Other Corporate Bonds | Market Approach | Minimum | Volatility of Credit    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
All Other Corporate Bonds - Private, Measurement Input 0.0500 0.0500
Fair Value, Inputs, Level 3 | All Other Corporate Bonds | Market Approach | Maximum | Volatility of Credit    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
All Other Corporate Bonds - Private, Measurement Input 0.0500 0.0500
Fair Value, Inputs, Level 3 | All Other Corporate Bonds | Market Approach | Weighted Average | Volatility of Credit    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
All Other Corporate Bonds - Private, Measurement Input 0.0500 0.0500
Fair Value, Inputs, Level 3 | Perpetual Preferred and Equity Securities | Market Approach    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Equity Securities [2] $ 26.8 $ 24.4
Fair Value, Inputs, Level 3 | Mortgage/Asset-backed Securities | Market Approach    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Debt Securities, Available-for-Sale [2],[3]   $ 21.2
[1] Represents various actuarial assumptions required to derive the liability cash flows. Fair value of embedded derivative is most often driven by the change in the weighted average credit spread to the swap curve for the assets backing the hypothetical loan
[2] Represents a decision to price based on par value, cost, owner's equity, or the price of the most recent capital funding round when limited data is available
[3] Represents basis point adjustments for credit-specific factors