| Cover Page | Oct. 29, 2020 |
| Cover [Abstract] | |
| Entity File Number | 0-50231 |
| City Area Code | 800 |
| Entity Tax Identification Number | 52-0883107 |
| Entity Address, Address Line One | 1100 15th Street, NW |
| Entity Incorporation, State or Country Code | X1 |
| Entity Central Index Key | 0000310522 |
| Document Type | 8-K |
| Document Period End Date | Oct. 29, 2020 |
| Entity Registrant Name | FEDERAL NATIONAL MORTGAGE ASSOCIATION FANNIE MAE |
| Local Phone Number | 232-6643 |
| Entity Address, City or Town | Washington, |
| Entity Address, State or Province | DC |
| Entity Address, Postal Zip Code | 20005 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
