Allowance for Loan Losses Rollforward by Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jan. 01, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	$ (13,209)	$ (13,232)	$ (9,016)	$ (14,203)
Transition impact of the adoption of the CECL standard	(12,966)		(12,966)
Allowance for loan losses for loans held for investment	(12,966)		(12,966)		$ (10,738)	$ (9,016)
Benefit (provision) for loan losses	161	1,212	(2,423)	1,846
Write-offs	45	562	134	943
Recoveries	(3)	(17)	(7)	(62)
Other	40	(7)	68	(6)
Ending Balance	(12,966)	(11,482)	(12,966)	(11,482)
Single-family [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	(12,070)	(12,985)	(8,759)	(13,969)
Allowance for loan losses for loans held for investment						(8,759)
Benefit (provision) for loan losses	389	1,239	(1,788)	1,886
Write-offs	46	558	116	939
Recoveries	(3)	(15)	(6)	(60)
Other	40	(7)	68	(6)
Ending Balance	(11,598)	(11,210)	(11,598)	(11,210)
Multifamily [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	(1,139)	(247)	(257)	(234)
Allowance for loan losses for loans held for investment						$ (257)
Benefit (provision) for loan losses	(228)	(27)	(635)	(40)
Write-offs	1	4	18	4
Recoveries	0	(2)	(1)	(2)
Ending Balance	$ (1,368)	$ (272)	$ (1,368)	$ (272)
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)