Outstanding Exposure - Net Direct Economic Exposure to Selected European Countries (Details) $ in Millions	Sep. 30, 2015 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	$ 2,234	[1]
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	866	[1]
Hungary [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	455	[1]
Hungary [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	385	[1]
Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	1,298	[1]
Italy [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	0	[1]
Portugal [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	89	[1]
Portugal [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	89	[1]
Spain [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	392	[1]
Spain [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	392	[1]
Total Sovereign Exposure [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	1,573	[1]
Total Sovereign Exposure [Member] | Non-Infrastructure Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	1,160	[1],[2]
Total Sovereign Exposure [Member] | Infrastructure finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	413	[1]
Total Sovereign Exposure [Member] | Hungary [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	279	[1]
Total Sovereign Exposure [Member] | Hungary [Member] | Non-Infrastructure Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	0	[1],[2]
Total Sovereign Exposure [Member] | Hungary [Member] | Infrastructure finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	279	[1]
Total Sovereign Exposure [Member] | Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	826	[1]
Total Sovereign Exposure [Member] | Italy [Member] | Non-Infrastructure Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	815	[1],[2]
Total Sovereign Exposure [Member] | Italy [Member] | Infrastructure finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	11	[1]
Total Sovereign Exposure [Member] | Portugal [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	89	[1]
Total Sovereign Exposure [Member] | Portugal [Member] | Non-Infrastructure Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	89	[1],[2]
Total Sovereign Exposure [Member] | Portugal [Member] | Infrastructure finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	0	[1]
Total Sovereign Exposure [Member] | Spain [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	379	[1]
Total Sovereign Exposure [Member] | Spain [Member] | Non-Infrastructure Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	256	[1],[2]
Total Sovereign Exposure [Member] | Spain [Member] | Infrastructure finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	123
Total Non-sovereign Exposure [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	661	[1]
Total Non-sovereign Exposure [Member] | Regulated utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	218	[1]
Total Non-sovereign Exposure [Member] | Residential Mortgage Backed Securities And Other Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	443	[1]
Total Non-sovereign Exposure [Member] | Hungary [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	176	[1]
Total Non-sovereign Exposure [Member] | Hungary [Member] | Regulated utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	0	[1]
Total Non-sovereign Exposure [Member] | Hungary [Member] | Residential Mortgage Backed Securities And Other Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	176	[1]
Total Non-sovereign Exposure [Member] | Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	472	[1]
Total Non-sovereign Exposure [Member] | Italy [Member] | Regulated utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	218	[1]
Total Non-sovereign Exposure [Member] | Italy [Member] | Residential Mortgage Backed Securities And Other Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	254	[1]
Total Non-sovereign Exposure [Member] | Portugal [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	0	[1]
Total Non-sovereign Exposure [Member] | Portugal [Member] | Regulated utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	0	[1]
Total Non-sovereign Exposure [Member] | Portugal [Member] | Residential Mortgage Backed Securities And Other Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	0	[1]
Total Non-sovereign Exposure [Member] | Spain [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	13	[1]
Total Non-sovereign Exposure [Member] | Spain [Member] | Regulated utilities [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	0
Total Non-sovereign Exposure [Member] | Spain [Member] | Residential Mortgage Backed Securities And Other Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, European Exposure	$ 13	[1]

[1] While the Company’s exposures are shown in U.S. dollars, the obligations the Company insures are in various currencies, primarily Euros. One of the residential mortgage-backed securities included in the table above includes residential mortgages in both Italy and Germany, and only the portion of the transaction equal to the portion of the original mortgage pool in Italian mortgages is shown in the table.

[2] The exposure shown in the “Non-infrastructure public finance” category is from transactions backed by receivable payments from sub-sovereigns in Italy, Spain and Portugal. Sub-sovereign debt is debt issued by a governmental entity or government backed entity, or supported by such an entity, that is other than direct sovereign debt of the ultimate governing body of the country.