Allowance for Loan Losses Rollforward by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jan. 01, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	$ (12,966)	$ (11,482)	$ (9,016)	$ (14,203)
Transition impact of the adoption of the CECL standard	(11,703)		(11,703)
Allowance for loan losses for loans held for investment	(11,703)		(11,703)		$ (10,738)	$ (9,016)
Benefit (provision) for loan losses	897	1,843	(1,526)	3,689
Write-offs	347	272	481	1,215
Recoveries	(4)	(9)	(11)	(71)
Other	23	0	91	(6)
Ending Balance	(11,703)	(9,376)	(11,703)	(9,376)
Single-family [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	(11,598)	(11,210)	(8,759)	(13,969)
Allowance for loan losses for loans held for investment						(8,759)
Benefit (provision) for loan losses	931	1,826	(857)	3,712
Write-offs	261	270	377	1,209
Recoveries	(4)	(8)	(10)	(68)
Other	23	0	91	(6)
Ending Balance	(10,387)	(9,122)	(10,387)	(9,122)
Multifamily [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	(1,368)	(272)	(257)	(234)
Allowance for loan losses for loans held for investment						$ (257)
Benefit (provision) for loan losses	(34)	17	(669)	(23)
Write-offs	86	2	104	6
Recoveries	0	(1)	(1)	(3)
Ending Balance	$ (1,316)	$ (254)	$ (1,316)	$ (254)
Accounting Standards Update 2016-13 [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses for loans held for investment					(1,722)
Accounting Standards Update 2016-13 [Member] | Single-family [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses for loans held for investment					(1,229)
Accounting Standards Update 2016-13 [Member] | Multifamily [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses for loans held for investment					$ (493)