Summary of Significant Accounting Policies Related Parties (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Jun. 30, 2021
Related Parties [Line Items]
Basis Points of Each Dollar of Unpaid Principal Balance	420.00%
Affordable Housing Program Obligation	$ 0.35
Temporary Payroll Tax Cut Continuation Act of 2011 (“TCCA”) fees	731,000,000	$ 637,000,000
Net income	4,993,000,000	461,000,000
US Treasury [Member]
Related Parties [Line Items]
Home Affordable Modification Program administrative expense reimbursements from Treasury and Freddie Mac	$ 4,000,000	5,000,000
Percentage of common shares attributable to warrants issued to US Treasury as percentage to total diluted common shares	79.90%
Increase of Guarantee Fee Rate Resulting from the Temporary Payroll Tax Cut Continuation Act of 2011.	1000.00%
Aggregate liquidation preference of senior preferred stock	$ 146,800,000,000
Total available funding from US Treasury pursuant to the senior preferred stock agreement	113,900,000,000
Aggregate funding received from US Treasury pursuant to the senior preferred stock purchase agreement	119,800,000,000
Temporary Payroll Tax Cut Continuation Act of 2011 (“TCCA”) fees	731,000,000	637,000,000
US Treasury [Member] | Forecast [Member]
Related Parties [Line Items]
Aggregate liquidation preference of senior preferred stock			$ 151,700,000,000
US Treasury [Member] | FNM_SingleFamily
Related Parties [Line Items]
Recognized TCCA fees that had not been remitted to Treasury as of period end	727,000,000
Federal Housing Finance Agency [Member]
Related Parties [Line Items]
FHFA assessment fees	37,000,000	32,000,000
Common Securitization Solutions [Member]
Related Parties [Line Items]
Payments to Acquire or Advance to Equity Method Investments	27,000,000	29,000,000
Other Expense [Member] | US Treasury [Member]
Related Parties [Line Items]
Affordable Housing Program Assessments	62,000,000	$ 30,000,000
Affordable housing program assessments not remitted	$ 62,000,000