Outstanding Exposure - Net Direct Economic Exposure to Selected European Countries (Details) $ in Millions	Mar. 31, 2017 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	$ 2,252
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	668
Hungary [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	318
Hungary [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	248
Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	1,312
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	345
Spain [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	345
Turkey [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	202
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	1,536
Total Sovereign Exposure [Member] | Hungary [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	204
Total Sovereign Exposure [Member] | Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	912
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	345
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	716
Total Non-sovereign Exposure [Member] | Hungary [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	114
Total Non-sovereign Exposure [Member] | Italy [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure	400
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	$ 202