Mortgage Loans and Loan Loss Reserves (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) numberofloans	Mar. 31, 2019 USD ($) numberofloans
Accounts, Notes, Loans and Financing Receivables [Line Items]
Noncash acquisition, mortgage loans held-for-investment acquired in exchange for issuance of debt securities of consolidated trusts	$ 73,200	$ 37,000
Transfers from advances to lenders to loans HFI	15,100	6,100
Credit to advances to lender cash path	$ 500	$ 400
Single-family
Accounts, Notes, Loans and Financing Receivables [Line Items]
Number of Loans, Modifications, Subsequent Default | numberofloans	2,816	4,325
Post-TDR Amortized Cost Basis, Modifications, Subsequent Default	$ 477	$ 463
Interest rate reduction and term extension types, percentage of completed modifications	14.00%	8.00%
Principal forebearance and interest rate reductions and term extension types, percentage of completed modifications	19.00%	23.00%
Average term extension, number of months of completed modifications	187 months	164 months
Average interest rate reduction, percentage of completed modifications	0.30%	0.10%
Multifamily
Accounts, Notes, Loans and Financing Receivables [Line Items]
Number of Loans, Modifications, Subsequent Default | numberofloans	0	0
Post-TDR Amortized Cost Basis, Modifications, Subsequent Default	$ 0	$ 0
Other loss mitigation activities
Accounts, Notes, Loans and Financing Receivables [Line Items]
Number of Loans, Modifications, Subsequent Default | numberofloans	1,026	1,464
Post-TDR Amortized Cost Basis, Modifications, Subsequent Default	$ 100	$ 200