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Concentrations of Credit Risk Geographic Concentration (Details) - Guaranty Book of Business [Member]
Dec. 31, 2016
Dec. 31, 2015
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% 19.00%
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% 16.00%
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 16.00% 17.00%
Multifamily [Member] | Southeast [Member]    
Concentration Risk [Line Items]    
Percentage of book of business 25.00% 23.00%
Multifamily [Member] | Southwest [Member]    
Concentration Risk [Line Items]    
Percentage of book of business 22.00% 21.00%
Multifamily [Member] | West [Member]    
Concentration Risk [Line Items]    
Percentage of book of business 28.00% 30.00%