Fair Value Measurement - Quantitative Information - Liabilities (Details) - Income Approach Valuation Technique [Member] - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Credit derivative liabilities, net [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Total liabilities carried at fair value	$ (361)	$ (389)
Credit derivative liabilities, net [Member] | Minimum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Internal floor (as a percent)	0.07%	0.07%
Bank profit (as a percent)	0.038%	0.038%
Hedge cost (as a percent)	0.038%	0.072%
Credit derivative liabilities, net [Member] | Minimum [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Year 1 loss estimates (as a percent)	0.00%	0.00%
Credit derivative liabilities, net [Member] | Maximum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Internal floor (as a percent)	1.00%	1.00%
Bank profit (as a percent)	8.25%	8.25%
Hedge cost (as a percent)	1.05%	1.181%
Credit derivative liabilities, net [Member] | Maximum [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Year 1 loss estimates (as a percent)	56.00%	38.00%
Credit derivative liabilities, net [Member] | Weighted Average [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Internal floor (as a percent)	0.212%	0.139%
Bank profit (as a percent)	0.735%	0.618%
Hedge cost (as a percent)	0.268%	0.245%
Credit derivative liabilities, net [Member] | Weighted Average [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Year 1 loss estimates (as a percent)	2.40%	1.30%
Financial Guaranty Variable Interest Entities [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Total liabilities carried at fair value	$ (820)	$ (958)
Financial Guaranty Variable Interest Entities [Member] | Minimum [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Conditional prepayment rate (as a percent)	3.50%	3.50%
Conditional default rate (as a percent)	2.50%	2.50%
Loss severity rate (as a percent)	55.00%	35.00%
Yield (as a percent)	2.40%	2.40%
Financial Guaranty Variable Interest Entities [Member] | Maximum [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Conditional prepayment rate (as a percent)	13.60%	12.00%
Conditional default rate (as a percent)	21.90%	21.60%
Loss severity rate (as a percent)	100.00%	100.00%
Yield (as a percent)	14.70%	20.00%
Financial Guaranty Variable Interest Entities [Member] | Weighted Average [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Conditional prepayment rate (as a percent)	9.00%	7.80%
Conditional default rate (as a percent)	5.60%	5.70%
Loss severity rate (as a percent)	78.20%	78.60%
Yield (as a percent)	4.90%	5.00%