Segment Reporting Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	$ 3,985,749	$ 3,503,319
Operating Segments [Member] | FNM_SingleFamily
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	3,569,130	3,149,212
Operating Segments [Member] | FNM_MultifamilyMember
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	$ 416,619	$ 354,107