Financial Guarantees Risk Characteristics - Multifamily (Details) (Multifamily [Member])	Jun. 30, 2013		Dec. 31, 2012
Guaranty Book of Business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of loans with detailed loan level information	99.00%		99.00%
Original Loan to Value Ratio Less Than or Equal to 80% [Member] | Guaranty Book of Business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	96.00%	[1],[2]	96.00%	[1],[2]
Loans seriously delinquent, percentage by unpaid principal balance	0.27%	[1],[3],[4]	0.24%	[1],[3],[4]
Original Loan to Value Ratio Greater Than 80% [Member] | Guaranty Book of Business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	4.00%	[1],[2]	4.00%	[1],[2]
Loans seriously delinquent, percentage by unpaid principal balance	0.41%	[1],[3],[4]	0.36%	[1],[3],[4]
Original Debt Service Coverage Ratio Less Than or Equal to 1.1 [Member] | Guaranty Book of Business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	7.00%	[1],[2]	8.00%	[1],[2]
Loans seriously delinquent, percentage by unpaid principal balance	1.43%	[1],[3],[4]	0.22%	[1],[3],[4]
Original Debt Service Coverage Ratio Greater Than 1.1 [Member] | Guaranty Book of Business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	93.00%	[1],[2]	92.00%	[1],[2]
Loans seriously delinquent, percentage by unpaid principal balance	0.19%	[1],[3],[4]	0.25%	[1],[3],[4]
Current Debt Service Coverage Ratio Less Than 1.0 [Member] | Guaranty Book of Business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	4.00%	[1],[2],[5]	5.00%	[1],[2],[5]
Loans seriously delinquent, percentage by unpaid principal balance	2.64%	[1],[3],[4],[5]	2.11%	[1],[3],[4],[5]
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Serious delinquency, days past due	60 days		60 days
Current debt service coverage ratio reporting lag	6 months		6 months
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current debt service coverage ratio reporting lag	12 months		12 months

[1] 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 June 30, 2013 and December 31, 2012 excluding loans that have been defeased.

[2] 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.

[3] Calculated based on the unpaid principal balance of multifamily loans that were seriously delinquent divided by the aggregate unpaid principal balance of multifamily loans for each category included in our guaranty book of business.

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

[5] Our estimates of current DSCRs are based on the latest available income information for these properties. Although we use the most recently available results of our multifamily borrowers, there is a lag in reporting, which typically can range from 6 to 12 months.