Comprehensive Text Block List (Policies)	3 Months Ended
Mar. 31, 2021
Text Block [Abstract]
Allowance for Loan Losses [Policy Text Block]	The estimated mark-to-market loan-to-value (“LTV”) ratio is a primary factor we consider when estimating our allowance for loan losses for single-family loans. As LTV ratios increase, the borrower's equity in the home decreases, which may negatively affect the borrower's ability to refinance or to sell the property for an amount at or above the outstanding balance of the loan.Property rental income and property valuations are key inputs to our internally assigned credit risk ratings.We maintain an allowance for loan losses for HFI loans held by Fannie Mae and by consolidated Fannie Mae MBS trusts, excluding loans for which we have elected the fair value option. When calculating our allowance for loan losses, we consider the unpaid principal balance, net of unamortized premiums and discounts, and other cost basis adjustments of HFI loans at the balance sheet date. We record write-offs as a reduction to our allowance for loan losses at the point of foreclosure, completion of a short sale, upon the redesignation of nonperforming and reperforming loans from HFI to HFS or when a loan is determined to be uncollectible.Our benefit or provision for loan losses can vary substantially from period to period based on a number of factors, such as changes in actual and forecasted home prices or property valuations, fluctuations in actual and forecasted interest rates, borrower payment behavior, events such as natural disasters or pandemics, the types and volume of our loss mitigation activities, including forbearance and loan modifications, the volume of foreclosures completed, and the redesignation of loans from HFI to HFS. Our benefit or provision can also be impacted by updates to the models, assumptions, and data used in determining our allowance for loan losses. As described below, our benefit or provision for loan losses and our loss reserves have been significantly affected by our estimates of the impact of the COVID-19 pandemic, which require significant management judgment. Changes in our estimates of borrowers that will ultimately receive forbearance and even more significantly, the loss mitigation outcomes of affected borrowers after the forbearance period ends, remain uncertain and can affect the amount of benefit or provision for loan losses we recognize.

Comprehensive Text Block List (Policies)

3 Months Ended

Mar. 31, 2021

Allowance for Loan Losses [Policy Text Block]

The estimated mark-to-market loan-to-value (“LTV”) ratio is a primary factor we consider when estimating our allowance for loan losses for single-family loans. As LTV ratios increase, the borrower's equity in the home decreases, which may negatively affect the borrower's ability to refinance or to sell the property for an amount at or above the outstanding balance of the loan.Property rental income and property valuations are key inputs to our internally assigned credit risk ratings.We maintain an allowance for loan losses for HFI loans held by Fannie Mae and by consolidated Fannie Mae MBS trusts, excluding loans for which we have elected the fair value option. When calculating our allowance for loan losses, we consider the unpaid principal balance, net of unamortized premiums and discounts, and other cost basis adjustments of HFI loans at the balance sheet date. We record write-offs as a reduction to our allowance for loan losses at the point of foreclosure, completion of a short sale, upon the redesignation of nonperforming and reperforming loans from HFI to HFS or when a loan is determined to be uncollectible.Our benefit or provision for loan losses can vary substantially from period to period based on a number of factors, such as changes in actual and forecasted home prices or property valuations, fluctuations in actual and forecasted interest rates, borrower payment behavior, events such as natural disasters or pandemics, the types and volume of our loss mitigation activities, including forbearance and loan modifications, the volume of foreclosures completed, and the redesignation of loans from HFI to HFS. Our benefit or provision can also be impacted by updates to the models, assumptions, and data used in determining our allowance for loan losses. As described below, our benefit or provision for loan losses and our loss reserves have been significantly affected by our estimates of the impact of the COVID-19 pandemic, which require significant management judgment. Changes in our estimates of borrowers that will ultimately receive forbearance and even more significantly, the loss mitigation outcomes of affected borrowers after the forbearance period ends, remain uncertain and can affect the amount of benefit or provision for loan losses we recognize.