Contracts Accounted for as Credit Derivatives - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Credit Derivatives
Estimated remaining weighted average life of credit derivatives (in years)	11 years 7 months 2 days	11 years 8 months
Net par outstanding	$ 6,182	$ 6,207
Collateral Debt Obligations, Maximum Posting Requirement [Member]
Credit Derivatives
Amount of par subject to collateral for which the amount of collateral is capped	250	464
Pooled infrastructure [Member]
Credit Derivatives
Net par outstanding	$ 1,373	$ 1,561