Outstanding Exposure - Net Direct Economic Exposure to Selected European Countries (Details) $ in Millions	Sep. 30, 2017 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	$ 2,553
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	793
Hungary [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	339
Hungary [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	262
Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	1,483
Italy [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	0
Portugal [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	75
Portugal [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	75
Spain [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	456
Spain [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	456
Turkey [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	200
Turkey [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	0
Total Sovereign Exposure [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	1,779
Total Sovereign Exposure [Member] | Hungary [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	214
Total Sovereign Exposure [Member] | Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	1,034
Total Sovereign Exposure [Member] | Portugal [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	75
Total Sovereign Exposure [Member] | Spain [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	456
Total Sovereign Exposure [Member] | Turkey [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	0
Total Non-sovereign Exposure [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	774
Total Non-sovereign Exposure [Member] | Hungary [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	125
Total Non-sovereign Exposure [Member] | Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	449
Total Non-sovereign Exposure [Member] | Portugal [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	0
Total Non-sovereign Exposure [Member] | Spain [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	0
Total Non-sovereign Exposure [Member] | Turkey [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	$ 200