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Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)
$ in Millions
Mar. 31, 2025
USD ($)
Dec. 31, 2024
USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Bonds available for sale $ 66,027 $ 64,006
Obligations of states, municipalities and political subdivisions    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Bonds available for sale $ 3,031 $ 3,143
Obligations of states, municipalities and political subdivisions | Minimum | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0511 0.0509
Obligations of states, municipalities and political subdivisions | Maximum | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0559 0.0557
Obligations of states, municipalities and political subdivisions | Weighted-average | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0535 0.0533
Obligations of states, municipalities and political subdivisions | Level 3    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Bonds available for sale $ 3 $ 3
Corporate debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Bonds available for sale $ 34,813 $ 31,826
Corporate debt | Minimum | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0598 0.0683
Corporate debt | Maximum | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.1104 0.1161
Corporate debt | Weighted-average | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0789 0.0922
Corporate debt | Level 3    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Bonds available for sale $ 57 $ 177
RMBS    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Bonds available for sale $ 9,052 $ 8,604
RMBS | Minimum | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0543 0.0589
RMBS | Minimum | Constant prepayment rate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0396 0.0410
RMBS | Minimum | Loss severity    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.3469 0.4081
RMBS | Minimum | Constant default rate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0060 0.0057
RMBS | Maximum | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0636 0.0698
RMBS | Maximum | Constant prepayment rate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0746 0.0926
RMBS | Maximum | Loss severity    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.8545 0.7672
RMBS | Maximum | Constant default rate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0197 0.0248
RMBS | Weighted-average | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0590 0.0644
RMBS | Weighted-average | Constant prepayment rate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0571 0.0668
RMBS | Weighted-average | Loss severity    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.6007 0.5876
RMBS | Weighted-average | Constant default rate    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0128 0.0152
RMBS | Level 3    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Bonds available for sale $ 1,251 $ 1,321
CLO/ABS | Minimum | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0333 0.0424
CLO/ABS | Maximum | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0911 0.0842
CLO/ABS | Weighted-average | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0622 0.0633
CLO/ABS | Level 3    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Bonds available for sale $ 760 $ 760
CMBS    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Bonds available for sale $ 3,565 $ 3,926
CMBS | Minimum | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0572 0.0704
CMBS | Maximum | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0894 0.1012
CMBS | Weighted-average | Yield    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Derivative asset, measurement input 0.0721 0.0870
CMBS | Level 3    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Bonds available for sale $ 5 $ 25