Financial Guarantees Risk Characteristics - Multifamily (Details) (Multifamily [Member])	Dec. 31, 2012		Dec. 31, 2011
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
Guaranty Book of Business [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	100.00%	[1],[2]	100.00%	[1],[2]
Percentage of loans with detailed loan level information	99.00%		99.00%
Guaranty Book of Business [Member] | Original Debt Service Coverage Ratio Less Than or Equal to 1.1 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	8.00%	[3],[4]	8.00%	[3],[4]
Seriously delinquent, percentage (loan count)	0.22%	[4],[5],[6]	0.24%	[4],[5],[6]
Guaranty Book of Business [Member] | Original Debt Service Coverage Ratio Greater Than 1.1 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	92.00%	[3],[4]	92.00%	[3],[4]
Seriously delinquent, percentage (loan count)	0.25%	[4],[5],[6]	0.62%	[4],[5],[6]
Guaranty Book of Business [Member] | Current Debt Service Coverage Ratio Less Than 1.0 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	5.00%	[3],[4],[7]	7.00%	[3],[4],[7]
Seriously delinquent, percentage (loan count)	2.11%	[4],[5],[6],[7]	3.66%	[4],[5],[6],[7]
Guaranty Book of Business [Member] | Original Loan to Value Ratio Greater Than 80% [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	4.00%	[3],[4]	5.00%	[3],[4]
Seriously delinquent, percentage (loan count)	0.36%	[4],[5],[6]	2.51%	[4],[5],[6]
Guaranty Book of Business [Member] | Original Loan to Value Ratio Less Than or Equal to 80% [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of book of business	96.00%	[3],[4]	95.00%	[3],[4]
Seriously delinquent, percentage (loan count)	0.24%	[4],[5],[6]	0.49%	[4],[5],[6]

[1] Midwest includes IL, IN, IA, MI, MN, NE, ND, OH, SD, WI; Northeast includes CT, DE, ME, MA, NH, NJ, NY, PA, PR, RI, VT, VI; Southeast includes AL, DC, FL, GA, KY, MD, MS, NC, SC, TN, VA, WV; Southwest includes AZ, AR, CO, KS, LA, MO, NM, OK, TX, UT; West includes AK, CA, GU, HI, ID, MT, NV, OR, WA and WY.

[2] Consists of the portion of our multifamily guaranty book of business for which we have detailed loan level information, which constituted 99% of our total multifamily guaranty book of business as of December 31, 2012 and 2011.

[3] 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.

[4] 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 December 31, 2012 and 2011 excluding loans that have been defeased.

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

[6] 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.

[7] 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.