Outstanding Exposure - Debt Service Outstanding (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	$ 397,233	$ 408,492
Net Debt Service Outstanding	390,017	401,118
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	382,557	393,010
Net Debt Service Outstanding	375,747	386,092
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	14,676	15,482
Net Debt Service Outstanding	$ 14,270	$ 15,026