Concentration of Credit and Other Risks - Concentration of Credit Risk - Single-Family Credit Guarantee Portfolio (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Concentration Risk [Line Items]
Serious delinquency rate	0.60%		0.63%
West
Concentration Risk [Line Items]
Serious delinquency rate	0.35%		0.36%
Northeast
Concentration Risk [Line Items]
Serious delinquency rate	0.83%		0.87%
North Central
Concentration Risk [Line Items]
Serious delinquency rate	0.59%		0.61%
Southeast
Concentration Risk [Line Items]
Serious delinquency rate	0.70%		0.73%
Southwest
Concentration Risk [Line Items]
Serious delinquency rate	0.52%		0.54%
ILLINOIS
Concentration Risk [Line Items]
Serious delinquency rate	0.82%		0.85%
NEW YORK
Concentration Risk [Line Items]
Serious delinquency rate	1.13%		1.21%
FLORIDA
Concentration Risk [Line Items]
Serious delinquency rate	0.71%		0.77%
NEW JERSEY
Concentration Risk [Line Items]
Serious delinquency rate	1.01%		1.08%
MARYLAND
Concentration Risk [Line Items]
Serious delinquency rate	0.82%		0.88%
All Other
Concentration Risk [Line Items]
Serious delinquency rate	0.52%		0.54%
Core single-family loan portfolio
Concentration Risk [Line Items]
Serious delinquency rate	0.26%		0.26%
Legacy and relief refinance single-family loan portfolio
Concentration Risk [Line Items]
Serious delinquency rate	1.79%		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 | MARYLAND
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	86.00%		85.00%
Single-family UPB | Legacy and relief refinance single-family loan portfolio
Concentration Risk [Line Items]
Concentration risk percentage	14.00%		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	8.00%	15.00%
Single-family Credit Losses | Northeast
Concentration Risk [Line Items]
Concentration risk percentage	36.00%	37.00%
Single-family Credit Losses | North Central
Concentration Risk [Line Items]
Concentration risk percentage	29.00%	16.00%
Single-family Credit Losses | Southeast
Concentration Risk [Line Items]
Concentration risk percentage	18.00%	25.00%
Single-family Credit Losses | Southwest
Concentration Risk [Line Items]
Concentration risk percentage	9.00%	7.00%
Single-family Credit Losses | ILLINOIS
Concentration Risk [Line Items]
Concentration risk percentage	16.00%	10.00%
Single-family Credit Losses | NEW YORK
Concentration Risk [Line Items]
Concentration risk percentage	9.00%	12.00%
Single-family Credit Losses | FLORIDA
Concentration Risk [Line Items]
Concentration risk percentage	9.00%	18.00%
Single-family Credit Losses | NEW JERSEY
Concentration Risk [Line Items]
Concentration risk percentage	8.00%	10.00%
Single-family Credit Losses | MARYLAND
Concentration Risk [Line Items]
Concentration risk percentage	5.00%	4.00%
Single-family Credit Losses | All Other
Concentration Risk [Line Items]
Concentration risk percentage	53.00%	46.00%
Single-family Credit Losses | Core single-family loan portfolio
Concentration Risk [Line Items]
Concentration risk percentage	21.00%	12.00%
Single-family Credit Losses | Legacy and relief refinance single-family loan portfolio
Concentration Risk [Line Items]
Concentration risk percentage	79.00%	88.00%