Credit Enhancements -Multifamily Credit Enhancements (Details) - MF Mortgage Loans Credit Enhancements - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Credit Enhancement [Line Items]
Maximum coverage	$ 43,476	$ 41,560
Multifamily | Non-consolidated VIE subordination
Credit Enhancement [Line Items]
Total current and protected UPB	271,946	251,008
Maximum coverage	42,008	40,262
Multifamily | Consolidated VIE subordination
Credit Enhancement [Line Items]
Total current and protected UPB	1,800	1,800
Maximum coverage	200	200
Multifamily | Lender-Risk Sharing
Credit Enhancement [Line Items]
Total current and protected UPB	3,347	2,529
Maximum coverage	579	381
Multifamily | Insurance/reinsurance
Credit Enhancement [Line Items]
Total current and protected UPB	5,399	2,769
Maximum coverage	191	127
Multifamily | SCR debt notes
Credit Enhancement [Line Items]
Total current and protected UPB	2,237	2,470
Maximum coverage	112	123
Multifamily | Other
Credit Enhancement [Line Items]
Total current and protected UPB	386	467
Maximum coverage	$ 386	$ 467