Concentrations of Credit Risk Risk Characteristics - SF (Details) (Single-Family [Member])	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
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%
Percentage of book of business	100.00%	100.00%
Seriously delinquent, percentage (loan count)	1.89%	2.38%
Year 2005 [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	3.00%	4.00%
Seriously delinquent, percentage (loan count)	6.18%	7.26%
Year 2006 [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	3.00%	3.00%
Seriously delinquent, percentage (loan count)	9.61%	11.26%
Year 2007 [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	4.00%	5.00%
Seriously delinquent, percentage (loan count)	10.79%	12.18%
Year 2008 [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	2.00%	3.00%
Seriously delinquent, percentage (loan count)	6.27%	6.69%
All Other Vintages [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	88.00%	85.00%
Seriously delinquent, percentage (loan count)	0.88%	1.02%
Alt-A [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	4.00%	5.00%
Seriously delinquent, percentage (loan count)	7.77%	9.23%
California | Guaranty Book of Business [Member] | Conventional Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	20.00%	20.00%
Seriously delinquent, percentage (loan count)	0.70%	0.98%
Florida | Guaranty Book of Business [Member] | Conventional Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	6.00%	6.00%
Seriously delinquent, percentage (loan count)	4.42%	6.89%
Illinois | Guaranty Book of Business [Member] | Conventional Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	4.00%	4.00%
Seriously delinquent, percentage (loan count)	2.36%	3.12%
New Jersey | Guaranty Book of Business [Member] | Conventional Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	4.00%	4.00%
Seriously delinquent, percentage (loan count)	5.78%	6.25%
New York | Guaranty Book of Business [Member] | Conventional Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	5.00%	5.00%
Seriously delinquent, percentage (loan count)	4.17%	4.42%
All other states [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	61.00%	61.00%
Seriously delinquent, percentage (loan count)	1.52%	1.85%
Estimated Mark-to-Market Loan-to-Value Ratio Greater Than 100% [Member] | Guaranty Book of Business [Member] | Conventional Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	5.00%	7.00%
Seriously delinquent, percentage (loan count)	10.98%	12.22%
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%