Fair Value Measurement - Quantitative Information - Liabilities (Details) - Valuation, Income Approach - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
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	$ (120)	$ (100)
Credit derivative liabilities, net | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Internal floor (as a percent)		0.15%
Bank profit (as a percent)	0.00%	0.47%
Hedge cost (as a percent)	0.135%	0.19%
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]
Internal floor (as a percent)		0.30%
Bank profit (as a percent)	2.917%	3.29%
Hedge cost (as a percent)	0.726%	0.99%
Credit derivative liabilities, net | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Year 1 loss estimates (as a percent)	79.10%	85.00%
Credit derivative liabilities, net | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Internal floor (as a percent)		0.21%
Bank profit (as a percent)	0.836%	0.93%
Hedge cost (as a percent)	0.227%	0.32%
Credit derivative liabilities, net | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Year 1 loss estimates (as a percent)	1.50%	1.90%
Financial Guaranty Variable Interest Entities | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total liabilities carried at fair value	$ (318)	$ (333)
Financial Guaranty Variable Interest Entities | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	1.60%	1.90%
Financial Guaranty Variable Interest Entities | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	5.90%	6.20%
Financial Guaranty Variable Interest Entities | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.10%	3.80%
Liabilities of CIVs | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total liabilities carried at fair value	$ (1,973)	$ (1,227)
Yield (as a percent)	1.60%	2.20%
Liabilities of CIVs | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	16.30%	15.20%
Liabilities of CIVs | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	2.30%	2.50%
Corporate securities | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	32.20%	42.00%
CPR | Financial Guaranty Variable Interest Entities | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	0.90%	0.90%
CPR | Financial Guaranty Variable Interest Entities | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	19.20%	19.00%
CPR | Financial Guaranty Variable Interest Entities | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	10.30%	9.40%
CDR | Financial Guaranty Variable Interest Entities | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	1.20%	1.90%
CDR | Financial Guaranty Variable Interest Entities | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	28.10%	26.60%
CDR | Financial Guaranty Variable Interest Entities | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	6.10%	6.00%
Loss severity | Financial Guaranty Variable Interest Entities | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	45.00%	45.00%
Loss severity | Financial Guaranty Variable Interest Entities | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	100.00%	100.00%
Loss severity | Financial Guaranty Variable Interest Entities | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	81.60%	81.50%