Contracts Accounted for as Credit Derivatives - Distribution of Credit Derivative Net Par Outstanding by Internal Rating (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Credit Derivatives
Net Par Outstanding	$ 5,615	$ 6,207
BIG [Member]
Credit Derivatives
Net Par Outstanding	258	338
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member]
Credit Derivatives
Net Par Outstanding	$ 5,615	$ 6,207
Percentage of installment premiums denominated in currencies other than the U.S. dollar	100.00%	100.00%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AAA [Member]
Credit Derivatives
Net Par Outstanding	$ 2,137	$ 2,144
Percentage of installment premiums denominated in currencies other than the U.S. dollar	38.10%	34.60%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, AA [Member]
Credit Derivatives
Net Par Outstanding	$ 1,053	$ 1,170
Percentage of installment premiums denominated in currencies other than the U.S. dollar	18.70%	18.80%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, A [Member]
Credit Derivatives
Net Par Outstanding	$ 1,203	$ 1,517
Percentage of installment premiums denominated in currencies other than the U.S. dollar	21.40%	24.50%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | Internal Credit Rating, BBB [Member]
Credit Derivatives
Net Par Outstanding	$ 964	$ 1,038
Percentage of installment premiums denominated in currencies other than the U.S. dollar	17.20%	16.70%
Credit Concentration Risk [Member] | Derivative, Aggregate Notional Amount [Member] | BIG [Member]
Credit Derivatives
Net Par Outstanding	$ 258	$ 338
Percentage of installment premiums denominated in currencies other than the U.S. dollar	4.60%	5.40%