Financial Guarantees Delinquency Status - Multifamily (Details) (Multifamily [Member])	Mar. 31, 2013		Dec. 31, 2012
Guaranty Book of Business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 days delinquent, percentage (unpaid principal balance)	0.07%	[1],[2]	0.23%	[1],[2]
Seriously delinquent, percentage (unpaid principal balance)	0.39%	[1],[2],[3]	0.24%	[1],[2],[3]
Percentage of loans with detailed loan level information	99.00%		99.00%
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Serious delinquency, days past due	60 days		60 days

[1] Calculated based on the aggregate unpaid principal balance of multifamily loans for each category divided by the aggregate unpaid principal balance of loans in our multifamily guaranty book of business.

[2] Consists of the portion of our multifamily guaranty book of business for which we have detailed loan level information, which constituted approximately 99% of our total multifamily guaranty book of business as of March 31, 2013 and December 31, 2012 excluding loans that have been defeased.

[3] Consists of multifamily loans that were 60 days or more past due as of the dates indicated.