Outstanding Exposure - Debt Service Outstanding (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	$ 408,492	$ 455,000
Net debt service outstanding	401,118	437,535
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	393,010	425,849
Net debt service outstanding	386,092	409,447
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	15,482	29,151
Net debt service outstanding	$ 15,026	$ 28,088