Derivative Instruments Derivatives 1 - Notional and FV Position (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Asset Derivatives [Abstract]
Notional amount	$ 329,466	$ 178,138
Derivative asset, estimated fair value	372	171
Netting adjustment	(2,608)	(4,272)
Liability Derivatives [Abstract]
Notional amount	238,457	338,089
Derivative liability, estimated fair value	(604)	(328)
Netting adjustment	2,756	4,979
Table Footnote [Abstract]
Cash collateral posted for derivative instruments	742	1,400
Cash collateral received for derivative instruments	594	649
Risk Management Derivatives
Asset Derivatives [Abstract]
Notional amount	237,307	108,750
Derivative asset, estimated fair value	2,274	3,476
Accrued interest receivable (payable)	418	835
Netting adjustment	(2,608)	(4,272)
Total net risk management derivatives	84	39
Liability Derivatives [Abstract]
Notional amount	91,860	229,864
Derivative liability, estimated fair value	(2,368)	(4,265)
Accrued interest receivable (payable)	(438)	(814)
Netting adjustment	2,756	4,979
Total net risk management derivatives	(50)	(100)
Mortgage commitment derivatives
Asset Derivatives [Abstract]
Notional amount	92,159	69,388
Derivative asset, estimated fair value	288	132
Netting adjustment	0	0
Liability Derivatives [Abstract]
Notional amount	146,597	108,225
Derivative liability, estimated fair value	(554)	(228)
Netting adjustment	0	0
Pay-fixed Swap
Asset Derivatives [Abstract]
Notional amount	93,391	52,732
Derivative asset, estimated fair value	723	772
Liability Derivatives [Abstract]
Notional amount	19,845	70,211
Derivative liability, estimated fair value	(674)	(2,120)
Receive-fixed Swap
Asset Derivatives [Abstract]
Notional amount	106,673	31,671
Derivative asset, estimated fair value	1,123	2,391
Liability Derivatives [Abstract]
Notional amount	63,209	138,852
Derivative liability, estimated fair value	(1,296)	(1,764)
Basis Swap
Asset Derivatives [Abstract]
Notional amount	273	873
Derivative asset, estimated fair value	98	124
Liability Derivatives [Abstract]
Notional amount	600	0
Derivative liability, estimated fair value	0	0
Foreign Currency Swap
Asset Derivatives [Abstract]
Notional amount	229	234
Derivative asset, estimated fair value	37	59
Liability Derivatives [Abstract]
Notional amount	231	236
Derivative liability, estimated fair value	(59)	(56)
Pay-fixed Swaption
Asset Derivatives [Abstract]
Notional amount	11,525	9,750
Derivative asset, estimated fair value	254	95
Liability Derivatives [Abstract]
Notional amount	600	4,000
Derivative liability, estimated fair value	(2)	(20)
Receive-fixed Swaption
Asset Derivatives [Abstract]
Notional amount	500	250
Derivative asset, estimated fair value	19	13
Liability Derivatives [Abstract]
Notional amount	7,375	9,250
Derivative liability, estimated fair value	(336)	(304)
Other
Asset Derivatives [Abstract]
Notional amount	24,716	13,240
Derivative asset, estimated fair value	20	22
Liability Derivatives [Abstract]
Notional amount	0	7,315
Derivative liability, estimated fair value	(1)	(1)
Mortgage commitments to purchase whole loans
Asset Derivatives [Abstract]
Notional amount	6,025	4,143
Derivative asset, estimated fair value	20	9
Liability Derivatives [Abstract]
Notional amount	1,506	1,570
Derivative liability, estimated fair value	(1)	(2)
Forward contracts to purchase mortgage-related securities
Asset Derivatives [Abstract]
Notional amount	80,636	45,925
Derivative asset, estimated fair value	263	108
Liability Derivatives [Abstract]
Notional amount	11,060	21,099
Derivative liability, estimated fair value	(16)	(21)
Forward contracts to sell mortgage-related securities
Asset Derivatives [Abstract]
Notional amount	5,498	19,320
Derivative asset, estimated fair value	5	15
Liability Derivatives [Abstract]
Notional amount	134,031	85,556
Derivative liability, estimated fair value	$ (537)	$ (205)