Outstanding Exposure - Debt Service Outstanding (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	$ 397,219	$ 408,492
Net Debt Service Outstanding	392,997	401,118
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	382,564	393,010
Net Debt Service Outstanding	378,746	386,092
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	14,655	15,482
Net Debt Service Outstanding	$ 14,251	$ 15,026