Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions		Sep. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	[1]	$ 65,980	$ 65,242
Obligations of states, municipalities and political subdivisions
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale		$ 4,651	$ 4,833
Obligations of states, municipalities and political subdivisions | Minimum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0460	0.0500
Obligations of states, municipalities and political subdivisions | Maximum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0482	0.0550
Obligations of states, municipalities and political subdivisions | Weighted-average | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0471	0.0523
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		$ 33,034	$ 32,346
Corporate debt | Minimum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0571	0.0516
Corporate debt | Maximum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.1024	0.0962
Corporate debt | Weighted-average | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0798	0.0739
Corporate debt | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale		$ 128	$ 332
RMBS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale		$ 6,419	$ 6,207
RMBS | Minimum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0523	0.0618
RMBS | Minimum | Constant prepayment rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0421	0.0443
RMBS | Minimum | Loss severity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.4159	0.4321
RMBS | Minimum | Constant default rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0080	0.0082
RMBS | Maximum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0640	0.0742
RMBS | Maximum | Constant prepayment rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0890	0.1030
RMBS | Maximum | Loss severity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.7742	0.7665
RMBS | Maximum | Constant default rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0238	0.0264
RMBS | Weighted-average | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0581	0.0680
RMBS | Weighted-average | Constant prepayment rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0655	0.0736
RMBS | Weighted-average | Loss severity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.5951	0.5993
RMBS | Weighted-average | Constant default rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0159	0.0173
RMBS | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale		$ 1,365	$ 1,341
CLO/ABS | Minimum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0310	0.0531
CLO/ABS | Maximum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0782	0.0856
CLO/ABS | Weighted-average | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0546	0.0694
CLO/ABS | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale		$ 1,003	$ 1,100
CMBS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale		$ 4,106	$ 4,147
CMBS | Minimum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.0507	0.0984
CMBS | Maximum | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.1574	0.1724
CMBS | Weighted-average | Yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, measurement input		0.1041	0.1354
CMBS | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale		$ 19	$ 22

[1] See Note 10 for details of balances associated with variable interest entities.