Outstanding Exposure - Net Direct Economic Exposure to Selected European Countries (Details) $ in Millions	Jun. 30, 2017 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	$ 2,365
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	709
Hungary [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	340
Hungary [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	265
Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	1,380
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	74
Portugal [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	74
Spain [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	370
Spain [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	370
Turkey [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	201
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	1,627
Total Sovereign Exposure [Member] | Hungary [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	218
Total Sovereign Exposure [Member] | Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	965
Total Sovereign Exposure [Member] | Portugal [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	74
Total Sovereign Exposure [Member] | Spain [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	370
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	738
Total Non-sovereign Exposure [Member] | Hungary [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	122
Total Non-sovereign Exposure [Member] | Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	415
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	$ 201