Allowance for Loan Losses Rollforward by Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Jan. 01, 2020
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	$ (10,552)	$ (9,016)
Transition impact of the adoption of the CECL standard	(9,628)
Allowance for loan losses for loans held for investment	(9,628)		$ (10,552)
Benefit (provision) for loan losses	766	(2,584)
Write-offs	104	89
Recoveries	(3)	(4)
Allowance for Loan and Lease Losses, Adjustments, Other	57	28
Ending Balance	(9,628)	(13,209)
Single-family [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	(9,344)	(8,759)
Benefit (provision) for loan losses	671	(2,177)
Write-offs	71	70
Recoveries	(2)	(3)
Allowance for Loan and Lease Losses, Adjustments, Other	57	28
Ending Balance	(8,547)	(12,070)
Multifamily [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Beginning balance	(1,208)	(257)
Benefit (provision) for loan losses	95	(407)
Write-offs	33	19
Recoveries	(1)	(1)
Ending Balance	$ (1,081)	$ (1,139)
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)