Fair Value Measurement - Quantitative Information - Liabilities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Credit derivative liabilities, net [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total liabilities carried at fair value	$ (262)	$ (185)
Credit derivative liabilities, net [Member] | Minimum [Member] | Valuation, Income Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Internal floor (as a percent)	0.09%		0.30%
Bank profit (as a percent)	0.00%		0.51%
Hedge cost (as a percent)	0.035%		0.05%
Credit derivative liabilities, net [Member] | Minimum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Year 1 loss estimates (as a percent)	0.00%	0.00%
Credit derivative liabilities, net [Member] | Maximum [Member] | Valuation, Income Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Internal floor (as a percent)	0.30%
Bank profit (as a percent)	3.69%		2.12%
Hedge cost (as a percent)	1.68%		0.31%
Credit derivative liabilities, net [Member] | Maximum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Year 1 loss estimates (as a percent)	85.00%	46.00%
Credit derivative liabilities, net [Member] | Weighted Average [Member] | Valuation, Income Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Internal floor (as a percent)	0.14%
Bank profit (as a percent)	1.04%		0.76%
Hedge cost (as a percent)	0.67%		0.11%
Credit derivative liabilities, net [Member] | Weighted Average [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Year 1 loss estimates (as a percent)	2.30%	1.30%
Financial Guaranty Variable Interest Entities [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total liabilities carried at fair value	$ (394)	$ (469)
Financial Guaranty Variable Interest Entities [Member] | Minimum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.20%	2.70%
Financial Guaranty Variable Interest Entities [Member] | Maximum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	10.80%	8.40%
Financial Guaranty Variable Interest Entities [Member] | Weighted Average [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	5.60%	4.20%
Collateralized Loan Obligations [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total liabilities carried at fair value	$ (426)	$ (481)
Yield (as a percent)	3.80%	10.00%
Collateralized Loan Obligations [Member] | Measurement Input, Discount Rate [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	29.00%
Assets of Consolidated Investment Vehicles [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)		12.50%
Assets of Consolidated Investment Vehicles [Member] | Minimum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.90%
Assets of Consolidated Investment Vehicles [Member] | Maximum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	21.20%
Assets of Consolidated Investment Vehicles [Member] | Weighted Average [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	12.70%
Measurement Input, Revenue Multiple [Member] | Assets of Consolidated Investment Vehicles [Member] | Minimum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	55.00%	50.00%
Measurement Input, Revenue Multiple [Member] | Assets of Consolidated Investment Vehicles [Member] | Maximum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	860.00%	670.00%
Measurement Input, Prepayment Rate [Member] | Financial Guaranty Variable Interest Entities [Member] | Minimum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	0.10%	0.10%
Measurement Input, Prepayment Rate [Member] | Financial Guaranty Variable Interest Entities [Member] | Maximum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	18.70%	18.60%
Measurement Input, Prepayment Rate [Member] | Financial Guaranty Variable Interest Entities [Member] | Weighted Average [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	8.40%	8.60%
Measurement Input, Default Rate [Member] | Financial Guaranty Variable Interest Entities [Member] | Minimum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	1.20%	1.20%
Measurement Input, Default Rate [Member] | Financial Guaranty Variable Interest Entities [Member] | Maximum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	24.90%	24.70%
Measurement Input, Default Rate [Member] | Financial Guaranty Variable Interest Entities [Member] | Weighted Average [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	4.90%	4.90%
Measurement Input, Loss Severity [Member] | Financial Guaranty Variable Interest Entities [Member] | Minimum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	40.00%	40.00%
Measurement Input, Loss Severity [Member] | Financial Guaranty Variable Interest Entities [Member] | Maximum [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
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 [Member] | Financial Guaranty Variable Interest Entities [Member] | Weighted Average [Member] | Valuation, Income Approach [Member] | Level 3 [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Transferor's continuing involvement, servicing assets or liabilities, prepayment speed	74.30%	76.10%