Contracts Accounted for as Credit Derivatives - Effect of Changes in Credit Spread (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Credit Risk Derivatives, 100 Percent Widening in Spreads, Effect on Fair Value	$ (734)
Credit Risk Derivatives, 50 Percent Widening in Spreads, Effect on Fair Value	(548)
Credit Risk Derivatives, 25 Percent Widening in Spreads, Effect on Fair Value	(455)
Credit Risk Derivatives, 10 Percent Widening in Spreads, Effect on Fair Value	(399)
Base Scenario	(361)	$ (389)
Credit Risk Derivatives, 10 Percent Narrowing in Spreads, Effect on Fair Value	(326)
Credit Risk Derivatives, 25 Percent Narrowing in Spreads, Effect on Fair Value	(274)
Credit Risk Derivatives, 50 Percent Narrowing in Spreads, Effect on Fair Value	(188)
Credit Risk Derivatives, 100 Percent Widening in Spreads, Effect on Unrealized Gain (Loss)	(373)
Credit Risk Derivatives, 50 Percent Widening in Spreads, Effect on Unrealized Gain (Loss)	(187)
Credit Risk Derivatives, 25 Percent Widening in Spreads, Effect on Unrealized Gain (Loss)	(94)
Credit Risk Derivatives, 10 Percent Widening in Spreads, Effect on Unrealized Gain (Loss)	(38)
Credit Risk Derivatives, Base Scenario, Effect on Unrealized Gain (Loss)	0
Credit Risk Derivatives, 10 Percent Narrowing in Spreads, Effect on Unrealized Gain (Loss)	35
Credit Risk Derivatives, 25 Percent Narrowing in Spreads, Effect on Unrealized Gain (Loss)	87
Credit Risk Derivatives, 50 Percent Narrowing in Spreads, Effect on Unrealized Gain (Loss)	$ 173