Outstanding Exposure - Debt Service Outstanding (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	$ 425,003	$ 455,000
Net Debt Service Outstanding	416,724	437,535
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	407,539	425,849
Net Debt Service Outstanding	399,347	409,447
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	17,464	29,151
Net Debt Service Outstanding	$ 17,377	$ 28,088