Fair Value Measurement - Quantitative Information - Liabilities (Details) - Valuation, Income Approach - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Credit derivative liabilities, net | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total liabilities carried at fair value	$ (159)	$ (185)
Credit derivative liabilities, net | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Internal floor (as a percent)		0.30%
Bank profit (as a percent)	0.48%	0.51%
Hedge cost (as a percent)	0.20%	0.05%
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]
Bank profit (as a percent)	2.85%	2.12%
Hedge cost (as a percent)	1.05%	0.31%
Credit derivative liabilities, net | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Year 1 loss estimates (as a percent)	85.00%	46.00%
Credit derivative liabilities, net | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Internal floor (as a percent)
Bank profit (as a percent)	1.01%	0.76%
Hedge cost (as a percent)	0.32%	0.11%
Credit derivative liabilities, net | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Year 1 loss estimates (as a percent)	2.00%	1.30%
Financial Guaranty Variable Interest Entities | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total liabilities carried at fair value	$ (355)	$ (469)
Financial Guaranty Variable Interest Entities | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	1.70%	2.70%
Financial Guaranty Variable Interest Entities | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	7.00%	8.40%
Financial Guaranty Variable Interest Entities | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.10%	4.20%
Liabilities of Consolidated Investment Vehicles | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total liabilities carried at fair value	$ (830)	$ (481)
Yield (as a percent)	2.30%	10.00%
Liabilities of Consolidated Investment Vehicles | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	12.80%
Liabilities of Consolidated Investment Vehicles | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	2.60%
Corporate securities | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	44.90%	35.90%
Measurement Input, Revenue Multiple | Corporate securities | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	66.00%	50.00%
Measurement Input, Revenue Multiple | Equity securities and warrants | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	66.00%	50.00%
Measurement Input, Prepayment Rate | Financial Guaranty Variable Interest Entities | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	0.90%	0.10%
Measurement Input, Prepayment Rate | Financial Guaranty Variable Interest Entities | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	18.90%	18.60%
Measurement Input, Prepayment Rate | Financial Guaranty Variable Interest Entities | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	8.70%	8.60%
Measurement Input, Default Rate | Financial Guaranty Variable Interest Entities | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	1.80%	1.20%
Measurement Input, Default Rate | Financial Guaranty Variable Interest Entities | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	25.60%	24.70%
Measurement Input, Default Rate | Financial Guaranty Variable Interest Entities | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	5.90%	4.90%
Measurement Input, Loss Severity | Financial Guaranty Variable Interest Entities | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	45.00%	40.00%
Measurement Input, Loss Severity | Financial Guaranty Variable Interest Entities | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	100.00%	100.00%
Measurement Input, Loss Severity | Financial Guaranty Variable Interest Entities | Weighted Average | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	79.90%	76.10%
Measurement Input, EBITDA Multiple | Corporate securities | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	100.00%	950.00%
Measurement Input, EBITDA Multiple | Equity securities and warrants | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed		950.00%
Measurement Input, EBITDA Multiple | Equity securities and warrants | Minimum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	800.00%
Measurement Input, EBITDA Multiple | Equity securities and warrants | Maximum | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	100.00%