Contracts Accounted for as Credit Derivatives - CDS Spread and Components of Credit Derivative Assets (Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Credit Derivatives
Fair value of credit derivatives before effect of AGC and AGM credit spreads	$ (555)	$ (811)
Plus: Effect of AGC and AGM credit spreads	286	422
Net fair value of credit derivatives	$ (269)	$ (389)
Credit Risk Contract, 5 Year Spread [Member] | AGC [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	1.63%	1.58%	3.76%
Credit Risk Contract, 5 Year Spread [Member] | AGM [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	1.45%	1.58%	3.66%
Credit Risk Contract, 1 Year Spread [Member] | AGC [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	0.70%	0.35%	1.39%
Credit Risk Contract, 1 Year Spread [Member] | AGM [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	0.28%	0.29%	1.31%