Outstanding Exposure - Debt Service Outstanding (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	$ 460,932	$ 455,000
Net Debt Service Outstanding	446,186	437,535
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	436,426	425,849
Net Debt Service Outstanding	422,496	409,447
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	24,506	29,151
Net Debt Service Outstanding	23,690	28,088
Excess Of Loss Reinsurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding	$ 444	$ 390