Derivative Instruments Derivatives 2 - FV Gains and 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
Derivative Instruments, Gain (Loss) [Line Items]
Derivative fair value gains (losses), net									$ 422	$ (1,185)	$ (835)
Net accrual of periodic settlements	$ 4,973	$ 5,369	$ 5,377	$ 5,232	$ 5,111	$ 5,274	$ 5,002	$ 5,346	20,951	20,733	21,295
Pay-fixed swap
Derivative Instruments, Gain (Loss) [Line Items]
Derivative fair value gains (losses), net									2,940	1,296	757
Receive-fixed Swap
Derivative Instruments, Gain (Loss) [Line Items]
Derivative fair value gains (losses), net									(1,834)	(851)	(751)
Basis swap
Derivative Instruments, Gain (Loss) [Line Items]
Derivative fair value gains (losses), net									(21)	21	(21)
Foreign currency swap
Derivative Instruments, Gain (Loss) [Line Items]
Derivative fair value gains (losses), net									(51)	49	(76)
Pay-fixed swaption
Derivative Instruments, Gain (Loss) [Line Items]
Derivative fair value gains (losses), net									100	(161)	163
Receive-fixed swaption
Derivative Instruments, Gain (Loss) [Line Items]
Derivative fair value gains (losses), net									(39)	(60)	(230)
Future [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative fair value gains (losses), net									38	22	154
Risk management derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative fair value gains (losses), net									72	(573)	(1,129)
Net accrual of periodic settlements									(1,061)	(889)	(1,125)
Mortgage commitment derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative fair value gains (losses), net									324	(603)	288
Credit enhancement derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative fair value gains (losses), net									$ 26	$ (9)	$ 6