Allowance for Loan Losses - Rollforward by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Jan. 01, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	$ (7,114)	$ (12,966)	$ (10,552)	$ (9,016)
Allowance for loan losses for HFI loans	(6,334)		(6,334)		$ (10,552)
Benefit (provision) for loan losses	884	897	4,178	(1,526)
Write-offs	58	347	325	481
Recoveries	(154)	(4)	(252)	(11)
Other	(8)	23	(33)	91
Ending Balance	(6,334)	(11,703)	(6,334)	(11,703)
Single-family
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	(6,064)	(11,598)	(9,344)	(8,759)
Benefit (provision) for loan losses	839	931	4,003	(857)
Write-offs	51	261	271	377
Recoveries	(133)	(4)	(212)	(10)
Other	(8)	23	(33)	91
Ending Balance	(5,315)	(10,387)	(5,315)	(10,387)
Multifamily
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	(1,050)	(1,368)	(1,208)	(257)
Benefit (provision) for loan losses	45	(34)	175	(669)
Write-offs	7	86	54	104
Recoveries	(21)	0	(40)	(1)
Ending Balance	$ (1,019)	$ (1,316)	$ (1,019)	$ (1,316)
Transition impact of the adoption of the CECL standard
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses for HFI loans						$ (1,722)
Transition impact of the adoption of the CECL standard | Single-family
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses for HFI loans						(1,229)
Transition impact of the adoption of the CECL standard | Multifamily
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses for HFI loans						$ (493)