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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Credit Derivatives
Realized gains on credit derivatives									$ 17	$ 56	$ 63
Net credit derivative losses (paid and payable) recovered and recoverable and other settlements									(27)	(27)	(81)
Realized gains (losses) and other settlements									(10)	29	(18)
Net change in unrealized gains (losses) on credit derivatives									121	69	746
Net change in fair value of credit derivatives	$ 5	$ 58	$ (6)	$ 54	$ 74	$ 21	$ 63	$ (60)	111	98	728
Pooled corporate obligations [Member]
Credit Derivatives
Net change in unrealized gains (losses) on credit derivatives									35	(16)	147
Pooled infrastructure [Member]
Credit Derivatives
Net change in unrealized gains (losses) on credit derivatives									5	17	17
Infrastructure finance [Member]
Credit Derivatives
Net change in unrealized gains (losses) on credit derivatives									4	4	0
Other [Member]
Credit Derivatives
Net change in unrealized gains (losses) on credit derivatives									54	42	186
United States [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Credit Derivatives
Net change in unrealized gains (losses) on credit derivatives									23	22	396
Terminated And Settlement Of Credit Derivative Contracts [Member]
Credit Derivatives
Net change in unrealized gains (losses) on credit derivatives									26	103	465
Net par of terminated credit derivative contracts									331	3,811	2,777
Realized gains on credit derivatives									0	20	13
Net credit derivative losses (paid and payable) recovered and recoverable and other settlements									$ (15)	$ 0	$ (116)