Concentrations of Credit Risk Geographic Concentration (Details) - Guaranty Book of Business [Member]	Dec. 31, 2020	Dec. 31, 2019
Single-Family [Member] | Conventional Mortgage [Member]
Concentration Risk [Line Items]
Percentage of book of business	100.00%	100.00%
Single-Family [Member] | Conventional Mortgage [Member] | Midwest [Member]
Concentration Risk [Line Items]
Percentage of book of business	14.00%	15.00%
Single-Family [Member] | Conventional Mortgage [Member] | Northeast [Member]
Concentration Risk [Line Items]
Percentage of book of business	17.00%	17.00%
Single-Family [Member] | Conventional Mortgage [Member] | Southeast [Member]
Concentration Risk [Line Items]
Percentage of book of business	22.00%	22.00%
Single-Family [Member] | Conventional Mortgage [Member] | Southwest [Member]
Concentration Risk [Line Items]
Percentage of book of business	19.00%	18.00%
Single-Family [Member] | Conventional Mortgage [Member] | West [Member]
Concentration Risk [Line Items]
Percentage of book of business	28.00%	28.00%
Multifamily [Member]
Concentration Risk [Line Items]
Percentage of book of business	100.00%	100.00%
Multifamily [Member] | Midwest [Member]
Concentration Risk [Line Items]
Percentage of book of business	11.00%	10.00%
Multifamily [Member] | Northeast [Member]
Concentration Risk [Line Items]
Percentage of book of business	15.00%	15.00%
Multifamily [Member] | Southeast [Member]
Concentration Risk [Line Items]
Percentage of book of business	27.00%	27.00%
Multifamily [Member] | Southwest [Member]
Concentration Risk [Line Items]
Percentage of book of business	22.00%	23.00%
Multifamily [Member] | West [Member]
Concentration Risk [Line Items]
Percentage of book of business	25.00%	25.00%