Fair Value Measurement - Quantitative Information - Liabilities (Details) - Income Approach Valuation Technique [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Credit derivative liabilities, net [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Total liabilities carried at fair value	$ (236)	$ (269)
Credit derivative liabilities, net [Member] | Minimum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Internal floor (as a percent)	0.088%	0.08%
Bank profit (as a percent)	0.083%	0.06%
Hedge cost (as a percent)	0.055%	0.176%
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)	0.30%	0.30%
Bank profit (as a percent)	5.159%	8.525%
Hedge cost (as a percent)	0.908%	1.226%
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)	43.00%	42.00%
Credit derivative liabilities, net [Member] | Weighted Average [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Internal floor (as a percent)	0.234%	0.218%
Bank profit (as a percent)	1.111%	1.75%
Hedge cost (as a percent)	0.34%	0.481%
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)	3.30%	3.30%
Financial Guaranty Variable Interest Entities [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Total liabilities carried at fair value	$ (708)	$ (757)
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)	1.70%	3.00%
Conditional default rate (as a percent)	1.30%	1.30%
Loss severity rate (as a percent)	60.00%	60.00%
Yield (as a percent)	3.70%	3.40%
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)	17.80%	14.90%
Conditional default rate (as a percent)	21.90%	21.70%
Loss severity rate (as a percent)	100.00%	100.00%
Yield (as a percent)	10.40%	10.00%
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)	9.20%	9.50%
Conditional default rate (as a percent)	5.20%	5.40%
Loss severity rate (as a percent)	79.70%	79.60%
Yield (as a percent)	5.20%	4.90%
Short-term Investments [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Asset, fair value		$ 1
Other invested assets [Member] | Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Asset, fair value	$ 7	$ 7