Summary of Significant Accounting Policies - Related Parties (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Related Parties [Line Items]
Senior preferred stock liquidation preference	$ 158,844		$ 158,844		$ 142,192
Temporary Payroll Tax Cut Continuation Act of 2011 (“TCCA”) fees	$ 781	$ 679	$ 2,270	$ 1,976
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%
Home Affordable Modification Program administrative expense reimbursements from Treasury and Freddie Mac	$ 4	5	$ 13	14
Increase in Single-family guarantee fees	0.10%		0.10%
Temporary Payroll Tax Cut Continuation Act of 2011 (“TCCA”) fees	$ 781	679	$ 2,270	1,976
US Treasury | Single-Family
Related Parties [Line Items]
Recognized TCCA fees that had not been remitted to Treasury as of period end	781		781
Federal Housing Finance Agency
Related Parties [Line Items]
FHFA assessment fees	35	35	107	102
Common Securitization Solutions
Related Parties [Line Items]
Capital contributed	15	19	60	67
Other Expense | US Treasury
Related Parties [Line Items]
Affordable Housing Program assessments recognized	$ 46	$ 59	165	$ 144
Affordable housing program assessments not remitted			$ 165