Concentrations of Credit Risk Geographic Concentration (Details) - Guaranty Book of Business [Member]	Dec. 31, 2017	Dec. 31, 2016
Single-Family [Member] | Conventional Mortgage [Member]
Concentration Risk [Line Items]
Percentage of book of business	100.00%	100.00%
Percentage of loans with detailed loan level information	99.00%	99.00%
Single-Family [Member] | Conventional Mortgage [Member] | Minimum [Member]
Concentration Risk [Line Items]
Percentage of loans with detailed loan level information	99.00%	99.00%
Single-Family [Member] | Conventional Mortgage [Member] | Midwest [Member]
Concentration Risk [Line Items]
Percentage of book of business	15.00%	15.00%
Single-Family [Member] | Conventional Mortgage [Member] | Northeast [Member]
Concentration Risk [Line Items]
Percentage of book of business	18.00%	18.00%
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	17.00%	17.00%
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%
Percentage of loans with detailed loan level information	99.00%	99.00%
Multifamily [Member] | Midwest [Member]
Concentration Risk [Line Items]
Percentage of book of business	9.00%	9.00%
Multifamily [Member] | Northeast [Member]
Concentration Risk [Line Items]
Percentage of book of business	15.00%	16.00%
Multifamily [Member] | Southeast [Member]
Concentration Risk [Line Items]
Percentage of book of business	26.00%	25.00%
Multifamily [Member] | Southwest [Member]
Concentration Risk [Line Items]
Percentage of book of business	23.00%	22.00%
Multifamily [Member] | West [Member]
Concentration Risk [Line Items]
Percentage of book of business	27.00%	28.00%