Contracts Accounted for as Credit Derivatives - CDS Spread and Components of Credit Derivative Assets (Liabilities) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Credit Derivatives
Fair value of credit derivatives before effect of AGC and AGM credit spreads	$ (499)	$ (555)
Plus: Effect of AGC and AGM credit spreads	263	286
Net fair value of credit derivatives	$ (236)	$ (269)
Credit Risk Contract, 5 Year Spread [Member] | AGC [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	1.21%	1.63%	1.73%	1.58%
Credit Risk Contract, 5 Year Spread [Member] | AGM [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	1.09%	1.45%	1.81%	1.58%
Credit Risk Contract, 1 Year Spread [Member] | AGC [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	0.25%	0.70%	0.31%	0.35%
Credit Risk Contract, 1 Year Spread [Member] | AGM [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	0.22%	0.28%	0.31%	0.29%