Concentration of Credit and Other Risks - Concentration of Credit Risk - Single-Family Credit Guarantee Portfolio (Details)	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Concentration Risk [Line Items]
Single-Family serious delinquency rate	3.04%		0.63%
West
Concentration Risk [Line Items]
Single-Family serious delinquency rate	2.84%		0.36%
Northeast
Concentration Risk [Line Items]
Single-Family serious delinquency rate	3.70%		0.87%
North Central
Concentration Risk [Line Items]
Single-Family serious delinquency rate	2.31%		0.61%
Southeast
Concentration Risk [Line Items]
Single-Family serious delinquency rate	3.42%		0.73%
Southwest
Concentration Risk [Line Items]
Single-Family serious delinquency rate	2.89%		0.54%
ILLINOIS
Concentration Risk [Line Items]
Single-Family serious delinquency rate	3.36%		0.85%
NEW YORK
Concentration Risk [Line Items]
Single-Family serious delinquency rate	5.26%		1.21%
FLORIDA
Concentration Risk [Line Items]
Single-Family serious delinquency rate	4.39%		0.77%
NEW JERSEY
Concentration Risk [Line Items]
Single-Family serious delinquency rate	5.09%		1.08%
PENNSYLVANIA
Concentration Risk [Line Items]
Single-Family serious delinquency rate	3.01%		0.89%
All Other
Concentration Risk [Line Items]
Single-Family serious delinquency rate	2.71%		0.53%
Core single-family loan portfolio
Concentration Risk [Line Items]
Single-Family serious delinquency rate	2.44%		0.26%
Legacy and relief refinance single-family loan portfolio
Concentration Risk [Line Items]
Single-Family serious delinquency rate	5.47%		1.84%
Single-family UPB
Concentration Risk [Line Items]
Concentration risk percentage	100.00%		100.00%
Single-family UPB | West
Concentration Risk [Line Items]
Concentration risk percentage	30.00%		30.00%
Single-family UPB | Northeast
Concentration Risk [Line Items]
Concentration risk percentage	24.00%		24.00%
Single-family UPB | North Central
Concentration Risk [Line Items]
Concentration risk percentage	16.00%		16.00%
Single-family UPB | Southeast
Concentration Risk [Line Items]
Concentration risk percentage	16.00%		16.00%
Single-family UPB | Southwest
Concentration Risk [Line Items]
Concentration risk percentage	14.00%		14.00%
Single-family UPB | ILLINOIS
Concentration Risk [Line Items]
Concentration risk percentage	4.00%		4.00%
Single-family UPB | NEW YORK
Concentration Risk [Line Items]
Concentration risk percentage	5.00%		5.00%
Single-family UPB | FLORIDA
Concentration Risk [Line Items]
Concentration risk percentage	6.00%		6.00%
Single-family UPB | NEW JERSEY
Concentration Risk [Line Items]
Concentration risk percentage	3.00%		3.00%
Single-family UPB | PENNSYLVANIA
Concentration Risk [Line Items]
Concentration risk percentage	3.00%		3.00%
Single-family UPB | All Other
Concentration Risk [Line Items]
Concentration risk percentage	79.00%		79.00%
Single-family UPB | Core single-family loan portfolio
Concentration Risk [Line Items]
Concentration risk percentage	89.00%		85.00%
Single-family UPB | Legacy and relief refinance single-family loan portfolio
Concentration Risk [Line Items]
Concentration risk percentage	11.00%		15.00%
Single-family Credit Losses
Concentration Risk [Line Items]
Concentration risk percentage	100.00%	100.00%
Single-family Credit Losses | West
Concentration Risk [Line Items]
Concentration risk percentage	5.00%	12.00%
Single-family Credit Losses | Northeast
Concentration Risk [Line Items]
Concentration risk percentage	40.00%	40.00%
Single-family Credit Losses | North Central
Concentration Risk [Line Items]
Concentration risk percentage	27.00%	19.00%
Single-family Credit Losses | Southeast
Concentration Risk [Line Items]
Concentration risk percentage	18.00%	22.00%
Single-family Credit Losses | Southwest
Concentration Risk [Line Items]
Concentration risk percentage	10.00%	7.00%
Single-family Credit Losses | ILLINOIS
Concentration Risk [Line Items]
Concentration risk percentage	14.00%	10.00%
Single-family Credit Losses | NEW YORK
Concentration Risk [Line Items]
Concentration risk percentage	12.00%	12.00%
Single-family Credit Losses | FLORIDA
Concentration Risk [Line Items]
Concentration risk percentage	10.00%	14.00%
Single-family Credit Losses | NEW JERSEY
Concentration Risk [Line Items]
Concentration risk percentage	9.00%	10.00%
Single-family Credit Losses | PENNSYLVANIA
Concentration Risk [Line Items]
Concentration risk percentage	5.00%	4.00%
Single-family Credit Losses | All Other
Concentration Risk [Line Items]
Concentration risk percentage	50.00%	50.00%
Single-family Credit Losses | Core single-family loan portfolio
Concentration Risk [Line Items]
Concentration risk percentage	33.00%	16.00%
Single-family Credit Losses | Legacy and relief refinance single-family loan portfolio
Concentration Risk [Line Items]
Concentration risk percentage	67.00%	84.00%