Summary of Significant Accounting Policies Related Parties (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Parties [Line Items]
Temporary Payroll Cut Continuation Act of 2011 (TCCA) fees	$ (697,000,000)	$ (679,000,000)	$ (660,000,000)	$ (637,000,000)	$ (626,000,000)	$ (613,000,000)	$ (600,000,000)	$ (593,000,000)		$ (2,673,000,000)	$ (2,432,000,000)	$ (2,284,000,000)
Basis Points of Each Dollar of Unpaid Principal Balance	420.00%									420.00%
Affordable Housing Program Obligation	$ 0.35									$ 0.35
Proceeds from Income Tax Refunds										4,000,000,000.0
Proceeds From Income Tax Refunds, Interest										$ 340,000,000
US Treasury [Member]
Related Parties [Line Items]
Percentage Of Common Shares Attributable to Warrants Issued to Treasury as Percentage to Total Diluted Common Shares	79.90%									79.90%
Aggregate Liquidation Preference of Senior Preferred Stock	$ 142,200,000,000									$ 142,200,000,000
Home Affordable Modification Program Administrative Expense Reimbursement										$ 19,000,000	20,000,000	24,000,000
Increase of Guarantee Fee Rate Resulting from the Temporary Payroll Tax Cut Continuation Act of 2011.	1000.00%									1000.00%
Temporary Payroll Cut Continuation Act of 2011 (TCCA) fees										$ (2,673,000,000)	(2,432,000,000)	(2,284,000,000)
US Treasury [Member] | Other Expense [Member]
Related Parties [Line Items]
Affordable Housing Program Assessments										211,000,000	98,000,000	75,000,000
us-gaap_AffordableHousingProgramAssessments										98,000,000	75,000,000
US Treasury [Member] | Forecast [Member] | Other Expense [Member]
Related Parties [Line Items]
Affordable Housing Program Assessments									$ 211,000,000
Federal Housing Finance Agency [Member]
Related Parties [Line Items]
FHFA assessment fees/expense										139,000,000	121,000,000	110,000,000
Common Securitization Solutions [Member]
Related Parties [Line Items]
Payments to Acquire Equity Method Investments										88,000,000	105,000,000	$ 135,000,000
FNM_SingleFamily | US Treasury [Member]
Related Parties [Line Items]
Recognized TCCA fees that had not been remitted to Treasury as of period end	$ 697,000,000				$ 626,000,000					$ 697,000,000	$ 626,000,000