Fannie Mae (FNMA)

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U.S. residential mortgage debt outstanding was estimated to be pproximately $12.6 trillion as of September 30, 2019 (the latest date for which information is available). We owned or guaranteed mortgage assets representing approximately 26% of total U.S. residential mortgage debt outstanding as of September 30, 2019.
(2019 fannie 10-K)

these two GSEs combined hold about one-third of the nation’s $16 trillion mortgage debt.