Contracts Accounted for as Credit Derivatives - Net Change in Fair Value of Credit Derivatives Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Credit Derivatives
Document Fiscal Year Focus									2018
Realized gains on credit derivatives									$ 9	$ 17	$ 56
Net credit derivative losses (paid and payable) recovered and recoverable and other settlements									(25)	(27)	(27)
Realized gains (losses) and other settlements									(16)	(10)	29
Net change in unrealized gains (losses) on credit derivatives									128	121	69
Net change in fair value of credit derivatives	$ 9	$ 21	$ 48	$ 34	$ 5	$ 58	$ (6)	$ 54	112	111	98
Terminated And Settlement Of Credit Derivative Contracts [Member]
Credit Derivatives
Net change in unrealized gains (losses) on credit derivatives									5	26	103
Net par of terminated credit derivative contracts									601	331	3,811
Realized gains on credit derivatives									$ 1	$ (15)	$ 20