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Concentrations of Credit Risk SF Risk Characteristics (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Single-Family [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
30 days delinquent (percentage of book of business) 1.42% 1.30%
60 days delinquent (percentage of book of business) 0.43% 0.36%
Seriously delinquent (percentage of book of business) 1.15% 1.18%
30 days delinquent (percentage of conventional loans) 1.63% 1.51%
60 days delinquent (percentage of conventional loans) 0.50% 0.41%
Seriously delinquent (percentage of conventional loans) 1.24% 1.20%
Percentage of book of business 100.00% 100.00%
Percentage of loans with detailed loan level information 99.00% 99.00%
Single-Family [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member] | California    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 0.42% 0.50%
Percentage of book of business 19.00% 19.00%
Single-Family [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member] | Florida    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 3.71% 1.89%
Percentage of book of business 6.00% 6.00%
Single-Family [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member] | New Jersey    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 2.15% 3.07%
Percentage of book of business 4.00% 4.00%
Single-Family [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member] | New York    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 2.02% 2.65%
Percentage of book of business 5.00% 5.00%
Single-Family [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member] | All other states    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 1.09% 1.11%
Percentage of book of business 66.00% 66.00%
Single-Family [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member] | Estimated mark-to-market LTV ratio greater than 100% [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 11.70% 10.44%
Percentage of book of business 1.00% 2.00%
Single-Family [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member] | Alt-A [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 4.95% 5.00%
Percentage of book of business 2.00% 3.00%
Single-Family [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member] | 2004 and Prior    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 3.28% 2.82%
Percentage of book of business 4.00% 5.00%
Single-Family [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member] | 2005 - 2008    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 6.55% 6.39%
Percentage of book of business 6.00% 8.00%
Single-Family [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member] | 2009 - 2017    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 0.53% 0.36%
Percentage of book of business 90.00% 87.00%
Single-Family [Member] | Minimum [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Serious delinquency: days past due 90 days 90 days
Single-Family [Member] | Minimum [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Percentage of loans with detailed loan level information 99.00% 99.00%
Multifamily [Member] | Guaranty Book of Business [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
30 days delinquent (percentage of book of business) 0.03% 0.02%
Seriously delinquent (percentage of book of business) 0.11% 0.05%
Percentage of book of business 100.00% 100.00%
Percentage of loans with detailed loan level information 99.00% 99.00%
Multifamily [Member] | Minimum [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Serious delinquency: days past due 60 days 60 days
Multifamily [Member] | Maximum [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Serious delinquency: days past due 89 days 89 days
Current debt service coverage ratio 1.0 1.0