Contracts Accounted for as Credit Derivatives - CDS Spread and Components of Credit Derivative Assets (Liabilities) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Credit Derivatives
Fair value of credit derivatives before effect of AGC and AGM credit spreads	$ (687)		$ (811)
Plus: Effect of AGC and AGM credit spreads	326		422
Net fair value of credit derivatives	$ (361)		$ (389)
Credit Risk Contract, 5 Year Spread [Member] | AGC [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	1.36%	1.73%	1.58%	2.65%	3.07%	3.76%
Credit Risk Contract, 5 Year Spread [Member] | AGM [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	1.40%	1.81%	1.58%	2.65%	3.09%	3.66%
Credit Risk Contract, 1 Year Spread [Member] | AGC [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	0.15%	0.31%	0.35%	0.45%	1.05%	1.39%
Credit Risk Contract, 1 Year Spread [Member] | AGM [Member]
Credit Derivatives
Quoted price of CDS contract (as a percent)	0.15%	0.31%	0.29%	0.47%	1.02%	1.31%