Outstanding Exposure - Puerto Rico Gross Par and Gross Debt Service Outstanding (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	$ 269,386	$ 307,474
Gross Debt Service Outstanding	408,492	455,000
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Par Outstanding	5,186	5,435
Gross Debt Service Outstanding	$ 8,514	$ 9,038