Concentrations of Credit Risk Concentration of Credit Risk (Narrative) (Details) (USD $)			3 Months Ended		9 Months Ended
	Sep. 30, 2012 insurers	Dec. 31, 2011	Sep. 30, 2012 Single-Family [Member]	Sep. 30, 2011 Single-Family [Member]	Sep. 30, 2012 Single-Family [Member]	Sep. 30, 2011 Single-Family [Member]	Sep. 30, 2012 Mortgage Sellers and Servicers [Member]	Dec. 31, 2011 Mortgage Sellers and Servicers [Member]	Sep. 30, 2012 Guaranty Book of Business [Member] Single-Family [Member] insurers servicers	Dec. 31, 2011 Guaranty Book of Business [Member] Single-Family [Member] servicers insurers	Sep. 30, 2012 Guaranty Book of Business [Member] Single-Family [Member] Insurance Service Provider Concentration Risk [Member]	Sep. 30, 2012 Guaranty Book of Business [Member] Single-Family [Member] Unpaid Principal Balance [Member] Credit Concentration Risk [Member] Largest Mortgage Servicers, Including Their Affiliates [Member]	Dec. 31, 2011 Guaranty Book of Business [Member] Single-Family [Member] Unpaid Principal Balance [Member] Credit Concentration Risk [Member] Largest Mortgage Servicers, Including Their Affiliates [Member]	Sep. 30, 2012 Guaranty Book of Business [Member] Single-Family [Member] Mortgage Insurance Coverage [Member] Insurance Service Provider Concentration Risk [Member]	Sep. 30, 2012 Guaranty Book of Business [Member] Single-Family [Member] Mortgage Insurance Coverage [Member] Insurance Service Provider Concentration Risk [Member] Mortgage Insurance Companies [Member]	Dec. 31, 2011 Guaranty Book of Business [Member] Single-Family [Member] Mortgage Insurance Coverage [Member] Insurance Service Provider Concentration Risk [Member] Mortgage Insurance Companies [Member]	Sep. 30, 2012 Guaranty Book of Business [Member] Multifamily [Member] servicers	Dec. 31, 2011 Guaranty Book of Business [Member] Multifamily [Member] servicers	Sep. 30, 2012 Guaranty Book of Business [Member] Multifamily [Member] Unpaid Principal Balance [Member] Credit Concentration Risk [Member] Largest Mortgage Servicers, Including Their Affiliates [Member]	Dec. 31, 2011 Guaranty Book of Business [Member] Multifamily [Member] Unpaid Principal Balance [Member] Credit Concentration Risk [Member] Largest Mortgage Servicers, Including Their Affiliates [Member]
Concentration of Credit Risk [Line Items]
Number of mortgage servicers, including their affiliates, which serviced the majority of our guaranty book of business									5	10							10	10
Percentage of concentration risk												59.00%	63.00%	78.00%	93.00%	94.00%			68.00%	67.00%
Number of Mortgage Insurers Publicly Disclosed Credit Quality Deterioration	6
Mortgage insurance coverage risk in force									$ 90,500,000,000	$ 91,200,000,000
Mortgage insurance coverage risk in force as percentage									3.00%
Primary mortgage insurance coverage risk in force									89,100,000,000	87,300,000,000
Pool mortgage insurance coverage risk in force									1,500,000,000	3,900,000,000
Number of mortgage insurance companies provided majority of our mortgage insurance									6	6
Mortgage insurance coverage risk in force, insurers with credit quality deterioration											70,400,000,000
Receivable from claims on insured defaulted loans	3,800,000,000	3,600,000,000					1,100,000,000	639,000,000
Allowance for mortgage insurance receivable	881,000,000	570,000,000
Proceeds received from primary and pool mortgage insurance policies			$ 1,100,000,000	$ 1,400,000,000	$ 3,600,000,000	$ 4,600,000,000