Derivative Instruments - Notional and FV Position (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Asset Derivatives
Notional Amount	$ 289,539	$ 450,981
Estimated Fair Value	809	1,225
Netting adjustment	(290)	(905)
Liability Derivatives
Notional Amount	160,048	288,198
Estimated Fair Value	(622)	(1,495)
Netting adjustment	838	995
Table Footnote [Abstract]
Cash collateral posted for derivative instruments	570	658
Cash collateral received for derivative instruments	22	568
Fair Value Hedging \| Designated as Hedging Instrument
Asset Derivatives
Notional Amount	43,408	0
Estimated Fair Value	0	0
Liability Derivatives
Notional Amount	205	0
Estimated Fair Value	(2)	0
Pay-fixed Swap
Asset Derivatives
Notional Amount	71,189	88,361
Estimated Fair Value	0	0
Liability Derivatives
Notional Amount	300	11,461
Estimated Fair Value	0	(595)
Pay-fixed Swap \| Fair Value Hedging \| Designated as Hedging Instrument
Asset Derivatives
Notional Amount	2,575	0
Estimated Fair Value	0	0
Liability Derivatives
Notional Amount	0	0
Estimated Fair Value	0	0
Receive-fixed Swap
Asset Derivatives
Notional Amount	26,348	92,315
Estimated Fair Value	1	233
Liability Derivatives
Notional Amount	44,273	33,919
Estimated Fair Value	(765)	(123)
Receive-fixed Swap \| Fair Value Hedging \| Designated as Hedging Instrument
Asset Derivatives
Notional Amount	40,833	0
Estimated Fair Value	0	0
Liability Derivatives
Notional Amount	205	0
Estimated Fair Value	(2)	0
Basis Swap
Asset Derivatives
Notional Amount	250	250
Estimated Fair Value	154	192
Liability Derivatives
Notional Amount	0	0
Estimated Fair Value	0	0
Foreign Currency Swap
Asset Derivatives
Notional Amount	233	237
Estimated Fair Value	28	57
Liability Derivatives
Notional Amount	101	239
Estimated Fair Value	(35)	(58)
Pay-fixed Swaption
Asset Derivatives
Notional Amount	4,430	5,530
Estimated Fair Value	68	37
Liability Derivatives
Notional Amount	160	2,025
Estimated Fair Value	(1)	(118)
Receive-fixed Swaption
Asset Derivatives
Notional Amount	390	3,355
Estimated Fair Value	7	346
Liability Derivatives
Notional Amount	700	700
Estimated Fair Value	(24)	(16)
Futures
Asset Derivatives
Notional Amount	30	64,398
Estimated Fair Value	0	0
Liability Derivatives
Notional Amount	0	0
Estimated Fair Value	0	0
Risk Management Derivatives
Asset Derivatives
Notional Amount	102,870	254,446
Estimated Fair Value	258	865
Accrued interest receivable (payable)	78	97
Netting adjustment	(290)	(905)
Liability Derivatives
Notional Amount	45,534	48,344
Estimated Fair Value	(825)	(910)
Accrued interest receivable (payable)	(14)	(105)
Netting adjustment	838	995
Risk Management Derivatives \| Not Designated as Hedging Instrument, Economic Hedge
Asset Derivatives
Notional Amount	102,870	254,446
Total net risk management derivatives	336	962
Liability Derivatives
Notional Amount	45,534	48,344
Total net risk management derivatives	(839)	(1,015)
Total risk management derivatives portfolio
Asset Derivatives
Notional Amount	146,278	254,446
Total net risk management derivatives	46	57
Liability Derivatives
Notional Amount	45,739	48,344
Total net risk management derivatives	(3)	(20)
Mortgage commitments to purchase whole loans
Asset Derivatives
Notional Amount	6,649	35,292
Estimated Fair Value	6	145
Liability Derivatives
Notional Amount	11,794	51
Estimated Fair Value	(71)	0
Forward contracts to purchase mortgage-related securities
Asset Derivatives
Notional Amount	11,360	144,215
Estimated Fair Value	22	844
Liability Derivatives
Notional Amount	73,742	607
Estimated Fair Value	(498)	0
Forward contracts to sell mortgage-related securities
Asset Derivatives
Notional Amount	115,159	199
Estimated Fair Value	735	0
Liability Derivatives
Notional Amount	20,606	227,828
Estimated Fair Value	(30)	(1,426)
Mortgage commitment derivatives
Asset Derivatives
Notional Amount	133,168	179,706
Estimated Fair Value	763	989
Netting adjustment	0	0
Liability Derivatives
Notional Amount	106,142	228,486
Estimated Fair Value	(599)	(1,426)
Netting adjustment	0	0
Credit enhancement derivatives
Asset Derivatives
Notional Amount	10,093	16,829
Estimated Fair Value	0	179
Liability Derivatives
Notional Amount	8,167	11,368
Estimated Fair Value	$ (20)	$ (49)

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