Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions	Sep. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 71,184	$ 64,006
Obligations of states, municipalities and political subdivisions
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 2,889	$ 3,143
Obligations of states, municipalities and political subdivisions | Minimum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0525	0.0509
Obligations of states, municipalities and political subdivisions | Maximum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0558	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	0.0541	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	$ 36,951	$ 31,826
Corporate debt | Minimum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0860	0.0683
Corporate debt | Maximum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.1676	0.1161
Corporate debt | Weighted-average | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.1363	0.0922
Corporate debt | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 20	$ 177
RMBS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 10,303	$ 8,604
RMBS | Minimum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0520	0.0589
RMBS | Minimum | Constant prepayment rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0405	0.0410
RMBS | Minimum | Loss severity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.3950	0.4081
RMBS | Minimum | Constant default rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0054	0.0057
RMBS | Maximum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0594	0.0698
RMBS | Maximum | Constant prepayment rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0751	0.0926
RMBS | Maximum | Loss severity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.7931	0.7672
RMBS | Maximum | Constant default rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0196	0.0248
RMBS | Weighted-average | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0557	0.0644
RMBS | Weighted-average | Constant prepayment rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0578	0.0668
RMBS | Weighted-average | Loss severity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.5941	0.5876
RMBS | Weighted-average | Constant default rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0125	0.0152
RMBS | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 1,197	$ 1,321
CLO/ABS | Minimum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0239	0.0424
CLO/ABS | Maximum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0954	0.0842
CLO/ABS | Weighted-average | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0588	0.0633
CLO/ABS | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 1,053	$ 760
CMBS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 4,190	$ 3,926
CMBS | Minimum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0556	0.0704
CMBS | Maximum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.1335	0.1012
CMBS | Weighted-average | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input	0.0878	0.0870
CMBS | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 5	$ 25