Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions	Mar. 31, 2026 USD ($)	Dec. 31, 2025 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 70,528	$ 71,032
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	3,290	3,451
Obligations of states, municipalities and political subdivisions
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 2,696	$ 2,775
Obligations of states, municipalities and political subdivisions | Minimum | Yield | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0533	0.0527
Obligations of states, municipalities and political subdivisions | Maximum | Yield | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0533	0.0527
Obligations of states, municipalities and political subdivisions | Weighted-average | Yield | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0533	0.0527
Obligations of states, municipalities and political subdivisions | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 4	$ 4
Obligations of states, municipalities and political subdivisions | Level 3 | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	2	2
RMBS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 11,072	$ 10,168
RMBS | Minimum | Yield | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0546	0.0525
RMBS | Minimum | Constant prepayment rate | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0395	0.0409
RMBS | Minimum | Loss severity | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.3741	0.3929
RMBS | Minimum | Constant default rate | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0049	0.0051
RMBS | Maximum | Yield | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0673	0.0630
RMBS | Maximum | Constant prepayment rate | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0719	0.0747
RMBS | Maximum | Loss severity | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.6498	0.7956
RMBS | Maximum | Constant default rate | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0187	0.0194
RMBS | Weighted-average | Yield | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0610	0.0577
RMBS | Weighted-average | Constant prepayment rate | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0557	0.0578
RMBS | Weighted-average | Loss severity | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.5119	0.5942
RMBS | Weighted-average | Constant default rate | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0118	0.0122
RMBS | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 1,486	$ 1,546
RMBS | Level 3 | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 1,135	$ 1,165
CLO/ABS | Minimum | Yield | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0440	0.0007
CLO/ABS | Maximum | Yield | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0582	0.1326
CLO/ABS | Weighted-average | Yield | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0511	0.0648
CLO/ABS | Level 3 | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 1,197	$ 1,321
CMBS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 4,636	$ 4,616
CMBS | Minimum | Yield | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0381	0.0495
CMBS | Maximum | Yield | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.1175	0.0495
CMBS | Weighted-average | Yield | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale, measurement input	0.0616	0.0495
CMBS | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 25	$ 24
CMBS | Level 3 | Discounted cash flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Bonds available for sale	$ 25	$ 24