Contracts Accounted for as Credit Derivatives - Net Fair Value and Expected Losses of Credit Derivatives by Sector (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Credit Risk Derivatives, at Fair Value, Net	$ (236)	$ (269)
Expected loss to be recovered	$ 6	$ 14