Allowance for Loan Losses - Rollforward by Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Jan. 01, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	$ (9,628)	$ (13,209)	$ (10,552)	$ (9,016)
Allowance for loan losses for HFI loans	(7,114)		(7,114)		$ (10,552)
Benefit (provision) for loan losses	2,528	161	3,294	(2,423)
Write-offs	163	45	267	134
Recoveries	(95)	(3)	(98)	(7)
Other	(82)	40	(25)	68
Ending Balance	(7,114)	(12,966)	(7,114)	(12,966)
Single-family
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	(8,547)	(12,070)	(9,344)	(8,759)
Benefit (provision) for loan losses	2,493	389	3,164	(1,788)
Write-offs	149	46	220	116
Recoveries	(77)	(3)	(79)	(6)
Other	(82)	40	(25)	68
Ending Balance	(6,064)	(11,598)	(6,064)	(11,598)
Multifamily
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	(1,081)	(1,139)	(1,208)	(257)
Benefit (provision) for loan losses	35	(228)	130	(635)
Write-offs	14	(1)	47	18
Recoveries	(18)	0	(19)	(1)
Ending Balance	$ (1,050)	$ (1,368)	$ (1,050)	$ (1,368)
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)