Outstanding Exposure - Debt Service Outstanding (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	$ 450,419	$ 455,000
Net Debt Service Outstanding	436,045	437,535
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	429,419	425,849
Net Debt Service Outstanding	415,689	409,447
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	21,000	29,151
Net Debt Service Outstanding	20,356	28,088
Excess Of Loss Reinsurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding	$ 506	$ 390