Mortgage Loans and Loan Loss Reserves (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) numberofloans	Sep. 30, 2019 USD ($) numberofloans	Sep. 30, 2020 USD ($) numberofloans	Sep. 30, 2019 USD ($) numberofloans
Accounts, Notes, Loans and Financing Receivables [Line Items]
Financing Receivable, Allowance for Credit Loss	$ 2,600		$ 2,600
Noncash acquisition, mortgage loans held-for-investment acquired in exchange for issuance of debt securities of consolidated trusts			293,900	$ 162,700
Transfers from advances to lenders to loans HFI			84,800	28,600
Credit to advances to lender cash path			$ 2,400	$ 1,600
Single-family
Accounts, Notes, Loans and Financing Receivables [Line Items]
Number of Loans, Modifications, Subsequent Default | numberofloans	2,213	3,563	9,750	11,644
Post-TDR Amortized Cost Basis, Modifications, Subsequent Default	$ 387	$ 443	$ 1,763	$ 1,367
Interest rate reduction and term extension types, percentage of completed modifications	16.00%	9.00%	15.00%	8.00%
Principal forbearance and interest rate reductions and term extension types, percentage of completed modifications	25.00%	23.00%	20.00%	24.00%
Average term extension, number of months of completed modifications	173 months	188 months	183 months	177 months
Average interest rate reduction, percentage of completed modifications	0.30%	0.20%	0.30%	0.10%
Multifamily
Accounts, Notes, Loans and Financing Receivables [Line Items]
Number of Loans, Modifications, Subsequent Default | numberofloans	0	0	0	0
Post-TDR Amortized Cost Basis, Modifications, Subsequent Default	$ 0	$ 0	$ 0	$ 0
Other loss mitigation activities
Accounts, Notes, Loans and Financing Receivables [Line Items]
Number of Loans, Modifications, Subsequent Default | numberofloans			2,903	3,983
Post-TDR Amortized Cost Basis, Modifications, Subsequent Default		$ 400	$ 400