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Concentrations of Credit Risk SF Risk Characteristics (Details) - Single-Family [Member]
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Guaranty Book of Business [Member] | Conventional Mortgage [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
30 days delinquent (percentage of book of business) 1.30% 1.27%
60 days delinquent (percentage of book of business) 0.36% 0.37%
Seriously delinquent (percentage of book of business) 1.18% 1.59%
30 days delinquent (percentage of conventional loans) 1.51% 1.46%
60 days delinquent (percentage of conventional loans) 0.41% 0.41%
Seriously delinquent (percentage of conventional loans) 1.20% 1.55%
Percentage of book of business 100.00% 100.00%
Percentage of loans with detailed loan level information 99.00% 99.00%
Guaranty Book of Business [Member] | Conventional Mortgage [Member] | California    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 0.50% 0.58%
Percentage of book of business 19.00% 20.00%
Guaranty Book of Business [Member] | Conventional Mortgage [Member] | Florida    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 1.89% 2.86%
Percentage of book of business 6.00% 6.00%
Guaranty Book of Business [Member] | Conventional Mortgage [Member] | New Jersey    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 3.07% 4.87%
Percentage of book of business 4.00% 4.00%
Guaranty Book of Business [Member] | Conventional Mortgage [Member] | New York    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 2.65% 3.55%
Percentage of book of business 5.00% 5.00%
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) 1.11% 1.34%
Percentage of book of business 66.00% 65.00%
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) 10.44% 10.76%
Percentage of book of business 2.00% 3.00%
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) 5.00% 6.53%
Percentage of book of business 3.00% 4.00%
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) 2.82% 3.06%
Percentage of book of business 5.00% 5.00%
Guaranty Book of Business [Member] | Conventional Mortgage [Member] | 2005 - 2008    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 6.39% 7.60%
Percentage of book of business 8.00% 10.00%
Guaranty Book of Business [Member] | Conventional Mortgage [Member] | 2009 - 2016    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Seriously delinquent (percentage of conventional loans) 0.36% 0.36%
Percentage of book of business 87.00% 85.00%
Minimum [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Serious delinquency: days past due 90 days 90 days
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%