Contracts Accounted for as Credit Derivatives - Effect of Changes in Credit Spread (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Credit Risk Derivatives, 100 Percent Widening in Spreads, Effect on Fair Value	$ (501)
Credit Risk Derivatives, 50 Percent Widening in Spreads, Effect on Fair Value	(385)
Credit Risk Derivatives, 25 Percent Widening in Spreads, Effect on Fair Value	(327)
Credit Risk Derivatives, 10 Percent Widening in Spreads, Effect on Fair Value	(292)
Fair value of credit derivative asset (liability), net	(269)	$ (389)
Credit Risk Derivatives, 10 Percent Narrowing in Spreads, Effect on Fair Value	(250)
Credit Risk Derivatives, 25 Percent Narrowing in Spreads, Effect on Fair Value	(222)
Credit Risk Derivatives, 50 Percent Narrowing in Spreads, Effect on Fair Value	(174)
Credit Risk Derivatives, 100 Percent Widening in Spreads, Effect on Unrealized Gain (Loss)	(232)
Credit Risk Derivatives, 50 Percent Widening in Spreads, Effect on Unrealized Gain (Loss)	(116)
Credit Risk Derivatives, 25 Percent Widening in Spreads, Effect on Unrealized Gain (Loss)	(58)
Credit Risk Derivatives, 10 Percent Widening in Spreads, Effect on Unrealized Gain (Loss)	(23)
Credit Risk Derivatives, Base Scenario, Effect on Unrealized Gain (Loss)	0
Credit Risk Derivatives, 10 Percent Narrowing in Spreads, Effect on Unrealized Gain (Loss)	19
Credit Risk Derivatives, 25 Percent Narrowing in Spreads, Effect on Unrealized Gain (Loss)	47
Credit Risk Derivatives, 50 Percent Narrowing in Spreads, Effect on Unrealized Gain (Loss)	$ 95