Fair Value Measurement - Quantitative Information - Liabilities (Details) - Valuation, Income Approach - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity method investments	$ 33
Credit derivative liabilities, net
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Internal floor (as a percent)	0.088%
Credit derivative liabilities, net | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total liabilities carried at fair value	$ (134)	$ (100)
Credit derivative liabilities, net | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Hedge cost (as a percent)	0.094%	0.19%
Bank profit (as a percent)	0.00%	0.47%
Internal floor (as a percent)		0.15%
Credit derivative liabilities, net | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Year 1 loss estimates (as a percent)	0.00%	0.00%
Credit derivative liabilities, net | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Hedge cost (as a percent)	0.461%	0.99%
Bank profit (as a percent)	1.869%	3.29%
Internal floor (as a percent)		0.30%
Credit derivative liabilities, net | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Year 1 loss estimates (as a percent)	85.80%	85.00%
Credit derivative liabilities, net | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Hedge cost (as a percent)	0.152%	0.32%
Bank profit (as a percent)	0.693%	0.93%
Internal floor (as a percent)		0.21%
Credit derivative liabilities, net | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Year 1 loss estimates (as a percent)	0.10%	1.90%
Financial Guaranty Variable Interest Entities | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total liabilities carried at fair value	$ (301)	$ (333)
Financial Guaranty Variable Interest Entities | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	1.50%	1.90%
Financial Guaranty Variable Interest Entities | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	7.90%	6.20%
Financial Guaranty Variable Interest Entities | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	3.90%	3.80%
Financial Guaranty Variable Interest Entities | CPR | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	0.90%	0.90%
Financial Guaranty Variable Interest Entities | CPR | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	21.70%	19.00%
Financial Guaranty Variable Interest Entities | CPR | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	12.20%	9.40%
Financial Guaranty Variable Interest Entities | CDR | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	1.40%	1.90%
Financial Guaranty Variable Interest Entities | CDR | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	26.90%	26.60%
Financial Guaranty Variable Interest Entities | CDR | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	7.90%	6.00%
Financial Guaranty Variable Interest Entities | Loss severity | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	45.00%	45.00%
Financial Guaranty Variable Interest Entities | Loss severity | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	100.00%	100.00%
Financial Guaranty Variable Interest Entities | Loss severity | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	81.60%	81.50%
CLO obligations of CFEs | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total liabilities carried at fair value	$ (2,937)	$ (1,227)
Yield (as a percent)	1.60%	2.20%
CLO obligations of CFEs | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	11.70%	15.20%
CLO obligations of CFEs | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	2.10%	2.50%
Warehouse financing debt | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total liabilities carried at fair value	$ (59)
Warehouse financing debt | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	12.70%
Warehouse financing debt | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	15.80%
Warehouse financing debt | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	14.10%
Other invested assets | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets carried at fair value	$ 7	$ 5