Consolidations and Transfers of Financial Assets (Policies)	12 Months Ended
	Dec. 31, 2018
Consolidations and Transfers of Financial Assets [Abstract]
Consolidations, Policy [Policy Text Block]	Principles of Consolidation Our consolidated financial statements include our accounts as well as the accounts of the other entities in which we have a controlling financial interest. All intercompany balances and transactions have been eliminated. The typical condition for a controlling financial interest is ownership of a majority of the voting interests of an entity. A controlling financial interest may also exist in entities through arrangements that do not involve voting interests, such as a variable interest entity (“VIE”). We have interests in various entities that are considered to be VIEs. The primary types of entities are securitization trusts guaranteed by us via lender swap and portfolio securitization transactions and mortgage-backed trusts that were not created by us, as well as housing partnerships that are established to finance the acquisition, construction, development or rehabilitation of affordable multifamily and single-family housing. These interests include investments in securities issued by VIEs, such as Fannie Mae MBS created pursuant to our securitization transactions and our guaranty to the entity. We consolidate the substantial majority of our single-class securitization trusts because our role as guarantor and master servicer provides us with the power to direct matters (primarily the servicing of mortgage loans) that impact the credit risk to which we are exposed. In contrast, we do not consolidate single-class securitization trusts when other organizations have the power to direct these activities.Unconsolidated VIEs We do not consolidate VIEs when we are not deemed to be the primary beneficiary.
Transfers and Servicing of Financial Assets, Policy [Policy Text Block]	Transfers of Financial Assets We evaluate a transfer of financial assets to determine whether the transfer qualifies as a sale. If the transfer does not meet the criteria for sale treatment, the transferred assets remain in our consolidated balance sheets and we record a liability to the extent of any proceeds received in connection with such a transfer. Transfers of financial assets for which we surrender control of the transferred assets are recorded as sales. When a transfer that qualifies as a sale is completed, we derecognize all assets transferred and recognize all assets obtained and liabilities incurred at fair value. The difference between the carrying basis of the assets transferred and the fair value of the proceeds from the sale is recorded as a component of “Investment gains, net” in our consolidated statements of operations and comprehensive income. Retained interests are primarily derived from transfers associated with our portfolio securitizations in the form of Fannie Mae MBS, REMIC certificates, guaranty assets and master servicing assets. When we receive a beneficial interest as a result of a resecuritization transaction, we classify this activity as non-cash activity in the consolidated statements of cash flows, which is not material to our consolidated financial statements. We separately describe the subsequent accounting, as well as how we determine fair value, for our retained interests in the Fannie Mae MBS included in the “Investments in Securities” section of this note. We enter into repurchase agreements that involve contemporaneous trades to purchase and sell securities. These transactions are accounted for as secured financings since the transferor has not relinquished control over the transferred assets. These transactions are reported as securities purchased under agreements to resell and securities sold under agreements to repurchase in our consolidated balance sheets except for securities purchased under agreements to resell on an overnight basis, which are included in cash and cash equivalents in our consolidated balance sheets.Transfers of Financial AssetsWe issue Fannie Mae MBS through portfolio securitization transactions by transferring pools of mortgage loans or mortgage-related securities to one or more trusts or special purpose entities. We are considered to be the transferor when we transfer assets from our own retained mortgage portfolio in a portfolio securitization transaction.

Disclosure of accounting policy for transfers and servicing financial assets, including securitization transactions as well as repurchase and resale agreements. This disclosure may include how the entity (1) determines whether a transaction is accounted for as a sale; (2) accounts for a sale transaction, including the initial and subsequent accounting for any interests that the entity obtains or continues to hold in the transaction, how such interests are valued, and the significant assumptions used in the valuation; (3) accounts for a transaction that does not qualify for sale treatment (that is, a financing); and (4) accounts for its servicing assets and liabilities ("servicing"), including how such servicing is measured initially and subsequently, and the methodology and significant assumptions used to value such servicing.