- Quantitative Information - Liabilities (Details) - Income Approach Valuation Technique [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Credit derivative liabilities, net [Member] | Minimum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Hedge Cost	0.176%	0.072%
Fair Value Inputs Bank Profit	0.06%	0.038%
Fair Value Inputs Internal Floor	0.08%	0.07%
Credit derivative liabilities, net [Member] | Maximum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Hedge Cost	1.226%	1.181%
Fair Value Inputs Bank Profit	8.525%	8.25%
Fair Value Inputs Internal Floor	0.30%	1.00%
Credit derivative liabilities, net [Member] | Weighted Average [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs Hedge Cost	0.481%	0.245%
Fair Value Inputs Bank Profit	1.075%	0.618%
Fair Value Inputs Internal Floor	0.218%	0.139%
Level 3 [Member] | Credit derivative liabilities, net [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Total liabilities carried at fair value	$ (269)	$ (389)
Level 3 [Member] | Credit derivative liabilities, net [Member] | Minimum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Year 1 loss estimates (as a percent)	0.00%	0.00%
Level 3 [Member] | Credit derivative liabilities, net [Member] | Maximum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Year 1 loss estimates (as a percent)	42.00%	38.00%
Level 3 [Member] | Credit derivative liabilities, net [Member] | Weighted Average [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Year 1 loss estimates (as a percent)	3.30%	1.30%
Level 3 [Member] | Financial Guaranty Variable Interest Entities [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Total liabilities carried at fair value	$ (757)	$ (958)
Level 3 [Member] | Financial Guaranty Variable Interest Entities [Member] | Minimum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Conditional prepayment rate (as a percent)	3.00%	3.50%
Conditional default rate (as a percent)	1.30%	2.50%
Loss severity rate (as a percent)	60.00%	35.00%
Yield (as a percent)	3.40%	2.40%
Level 3 [Member] | Financial Guaranty Variable Interest Entities [Member] | Maximum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Conditional prepayment rate (as a percent)	14.90%	12.00%
Conditional default rate (as a percent)	21.70%	21.60%
Loss severity rate (as a percent)	100.00%	100.00%
Yield (as a percent)	10.00%	20.00%
Level 3 [Member] | Financial Guaranty Variable Interest Entities [Member] | Weighted Average [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Conditional prepayment rate (as a percent)	9.50%	7.80%
Conditional default rate (as a percent)	5.40%	5.70%
Loss severity rate (as a percent)	79.60%	78.60%
Yield (as a percent)	4.90%	5.00%