Comprehensive Text Block List (Policies)	9 Months Ended
Sep. 30, 2020
Text Block [Abstract]
Allowance for Loan Losses [Policy Text Block]	Allowance for Loan Losses Our allowance for loan losses is a valuation account that is deducted from the amortized cost basis of HFI loans to present the net amount expected to be collected on the loans. The allowance for loan losses reflects an estimate of expected credit losses on single-family and multifamily HFI loans held by Fannie Mae and by consolidated Fannie Mae MBS trusts. Estimates of credit losses are based on expected cash flows derived from internal models that estimate loan performance under simulated ranges of economic environments. Our modeled loan performance is based on our historical experience of loans with similar risk characteristics adjusted to reflect current conditions and reasonable and supportable forecasts. Our historical loss experience and our credit loss estimates capture the possibility of remote events that could result in credit losses on loans that are considered low risk. The allowance for loan losses does not consider benefits from freestanding credit enhancements, such as our CAS and CIRT programs and multifamily DUS lender risk-sharing arrangements, which are recorded in “Other assets” in our condensed consolidated balance sheets. For loans that were current as of March 1, 2020 and subsequently became delinquent or entered into forbearance during the COVID-19 pandemic, see changes to our allowance for loan losses in the section on New Accounting Guidance below. For loans not subject to the COVID-19 related guidance, we have elected not to measure an allowance for credit losses on accrued interest receivable balances as we have a nonaccrual policy to ensure the timely reversal of unpaid accrued interest. See “Note 4, Allowance for Loan Losses” for additional information about our current period provision for loan losses, including a discussion of the estimates used in measuring the impact of the COVID-19 pandemic on our allowance. Changes to our estimate of expected credit losses, including changes due to the passage of time, are recorded through the benefit (provision) for credit losses. When calculating our allowance for loan losses, we consider only our amortized cost in the loans at the balance sheet date. We record write-offs as a reduction to the allowance for loan losses when losses are confirmed through the receipt of assets in satisfaction of a loan, such as the underlying collateral upon foreclosure or cash upon completion of a short sale. Additionally, we record write-offs as a reduction to our allowance for loan losses when a loan is determined to be uncollectible and upon the transfer of a nonperforming loan from HFI to HFS. We include expected recoveries of amounts previously written off and expected to be written off in determining our allowance for loan losses.The estimated mark-to-market 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 loanProperty 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, during 2020, 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. As a result of our update to the application of our nonaccrual policy in the second quarter of 2020, we continue to accrue interest for single-family and multifamily loans that were negatively impacted by the COVID-19 pandemic. This update resulted in a significant portion of delinquent loans, that were current as of March 1, 2020 and subsequently became delinquent, remaining on accrual status. Accordingly, we established a valuation allowance for expected credit losses on the accrued interest receivable balance based on our evaluation of collectability.(2)

Comprehensive Text Block List (Policies)

9 Months Ended

Sep. 30, 2020

Allowance for Loan Losses [Policy Text Block]

Allowance for Loan Losses

Our allowance for loan losses is a valuation account that is deducted from the amortized cost basis of HFI loans to present the net amount expected to be collected on the loans. The allowance for loan losses reflects an estimate of expected credit losses on single-family and multifamily HFI loans held by Fannie Mae and by consolidated Fannie Mae MBS trusts. Estimates of credit losses are based on expected cash flows derived from internal models that estimate loan performance under simulated ranges of economic environments. Our modeled loan performance is based on our historical experience of loans with similar risk characteristics adjusted to reflect current conditions and reasonable and supportable forecasts. Our historical loss experience and our credit loss estimates capture the possibility of remote events that could result in credit losses on loans that are considered low risk. The allowance for loan losses does not consider benefits from freestanding credit enhancements, such as our CAS and CIRT programs and multifamily DUS lender risk-sharing arrangements, which are recorded in “Other assets” in our condensed consolidated balance sheets. For loans that were current as of March 1, 2020 and subsequently became delinquent or entered into forbearance during the COVID-19 pandemic, see changes to our allowance for loan losses in the section on New Accounting Guidance below. For loans not subject to the COVID-19 related guidance, we have elected not to measure an allowance for credit losses on accrued interest receivable balances as we have a nonaccrual policy to ensure the timely reversal of unpaid accrued interest. See “Note 4, Allowance for Loan Losses” for additional information about our current period provision for loan losses, including a discussion of the estimates used in measuring the impact of the COVID-19 pandemic on our allowance.

Changes to our estimate of expected credit losses, including changes due to the passage of time, are recorded through the benefit (provision) for credit losses. When calculating our allowance for loan losses, we consider only our amortized cost in the loans at the balance sheet date. We record write-offs as a reduction to the allowance for loan losses when losses are confirmed through the receipt of assets in satisfaction of a loan, such as the underlying collateral upon foreclosure or cash upon completion of a short sale. Additionally, we record write-offs as a reduction to our allowance for loan losses when a loan is determined to be uncollectible and upon the transfer of a nonperforming loan from HFI to HFS. We include expected recoveries of amounts previously written off and expected to be written off in determining our allowance for loan losses.The estimated mark-to-market 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 loanProperty 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, during 2020, 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. As a result of our update to the application of our nonaccrual policy in the second quarter of 2020, we continue to accrue interest for single-family and multifamily loans that were negatively impacted by the COVID-19 pandemic. This update resulted in a significant portion of delinquent loans, that were current as of March 1, 2020 and subsequently became delinquent, remaining on accrual status. Accordingly, we established a valuation allowance for expected credit losses on the accrued interest receivable balance based on our evaluation of collectability.(2)