Derivative Instruments Derivatives 1 - Notional and FV Position (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Asset Derivatives [Abstract]
Notional amount	$ 336,556	$ 178,138
Derivative asset, estimated fair value	353	171
Netting adjustment	(2,775)	(4,272)
Liability Derivatives [Abstract]
Notional amount	153,641	338,089
Derivative liability, estimated fair value	(276)	(328)
Netting adjustment	2,648	4,979
Table Footnote [Abstract]
Cash collateral posted for derivative instruments	611	1,400
Cash collateral received for derivative instruments	738	649
Pay-fixed Swap
Asset Derivatives [Abstract]
Notional amount	103,362	52,732
Derivative asset, estimated fair value	898	772
Liability Derivatives [Abstract]
Notional amount	18,245	70,211
Derivative liability, estimated fair value	(520)	(2,120)
Receive-fixed Swap
Asset Derivatives [Abstract]
Notional amount	114,773	31,671
Derivative asset, estimated fair value	1,031	2,391
Liability Derivatives [Abstract]
Notional amount	60,219	138,852
Derivative liability, estimated fair value	(1,259)	(1,764)
Basis Swap
Asset Derivatives [Abstract]
Notional amount	273	873
Derivative asset, estimated fair value	93	124
Liability Derivatives [Abstract]
Notional amount	600	0
Derivative liability, estimated fair value	0	0
Foreign Currency Swap
Asset Derivatives [Abstract]
Notional amount	225	234
Derivative asset, estimated fair value	28	59
Liability Derivatives [Abstract]
Notional amount	228	236
Derivative liability, estimated fair value	(60)	(56)
Pay-fixed Swaption
Asset Derivatives [Abstract]
Notional amount	11,775	9,750
Derivative asset, estimated fair value	321	95
Liability Derivatives [Abstract]
Notional amount	350	4,000
Derivative liability, estimated fair value	(2)	(20)
Receive-fixed Swaption
Asset Derivatives [Abstract]
Notional amount	500	250
Derivative asset, estimated fair value	14	13
Liability Derivatives [Abstract]
Notional amount	7,375	9,250
Derivative liability, estimated fair value	(366)	(304)
Other
Asset Derivatives [Abstract]
Notional amount	25,799	13,240
Derivative asset, estimated fair value	18	22
Liability Derivatives [Abstract]
Notional amount	0	7,315
Derivative liability, estimated fair value	(1)	(1)
Risk Management Derivatives
Asset Derivatives [Abstract]
Notional amount	256,707	108,750
Derivative asset, estimated fair value	2,403	3,476
Accrued interest receivable (payable)	437	835
Netting adjustment	(2,775)	(4,272)
Total net risk management derivatives	65	39
Liability Derivatives [Abstract]
Notional amount	87,017	229,864
Derivative liability, estimated fair value	(2,208)	(4,265)
Accrued interest receivable (payable)	(504)	(814)
Netting adjustment	2,648	4,979
Total net risk management derivatives	(64)	(100)
Mortgage commitments to purchase whole loans
Asset Derivatives [Abstract]
Notional amount	2,569	4,143
Derivative asset, estimated fair value	3	9
Liability Derivatives [Abstract]
Notional amount	3,054	1,570
Derivative liability, estimated fair value	(15)	(2)
Forward contracts to purchase mortgage-related securities
Asset Derivatives [Abstract]
Notional amount	15,301	45,925
Derivative asset, estimated fair value	21	108
Liability Derivatives [Abstract]
Notional amount	36,797	21,099
Derivative liability, estimated fair value	(149)	(21)
Forward contracts to sell mortgage-related securities
Asset Derivatives [Abstract]
Notional amount	61,979	19,320
Derivative asset, estimated fair value	264	15
Liability Derivatives [Abstract]
Notional amount	26,773	85,556
Derivative liability, estimated fair value	(48)	(205)
Mortgage commitment derivatives
Asset Derivatives [Abstract]
Notional amount	79,849	69,388
Derivative asset, estimated fair value	288	132
Netting adjustment	0	0
Liability Derivatives [Abstract]
Notional amount	66,624	108,225
Derivative liability, estimated fair value	(212)	(228)
Netting adjustment	$ 0	$ 0