Outstanding Exposure - Debt Service Outstanding (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	$ 367,696	$ 375,080
Net Debt Service Outstanding	364,641	371,586
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	354,406	361,511
Net Debt Service Outstanding	351,396	358,438
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Gross Debt Service Outstanding	13,290	13,569
Net Debt Service Outstanding	$ 13,245	$ 13,148