Summary of Significant Accounting Policies - Related Parties (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Related Parties [Line Items]
Temporary Payroll Tax Cut Continuation Act of 2011 (“TCCA”) fees	$ 758	$ 660	$ 1,489	$ 1,297
Basis Points of Each Dollar of Unpaid Principal Balance	0.042%		0.042%
Affordable Housing Program obligation payable to Treasury's Capital Magnet Fund (percent)	35.00%		35.00%
US Treasury
Related Parties [Line Items]
Percentage of common shares attributable to warrants issued to US Treasury as percentage to total diluted common shares	79.90%		79.90%
Aggregate liquidation preference of senior preferred stock	$ 151,700		$ 151,700
Home Affordable Modification Program administrative expense reimbursements from Treasury and Freddie Mac	$ 5	4	$ 9	9
Increase in Single-family guarantee fees	0.10%		0.10%
Temporary Payroll Tax Cut Continuation Act of 2011 (“TCCA”) fees	$ 758	660	$ 1,489	1,297
US Treasury | Single-Family
Related Parties [Line Items]
Recognized TCCA fees that had not been remitted to Treasury as of period end	758		758
Federal Housing Finance Agency [Member]
Related Parties [Line Items]
FHFA assessment fees	35	35	72	67
Common Securitization Solutions [Member]
Related Parties [Line Items]
Capital contributed	18	19	45	48
Other Expense [Member] | US Treasury
Related Parties [Line Items]
Affordable Housing Program assessments recognized	$ 57	$ 55	119	$ 85
Affordable housing program assessments not remitted			$ 119