Outstanding Exposure - Puerto Rico Net Par Outstanding (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 257,089	$ 264,952
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,967	4,966
Constitutionally Guaranteed [Member] | Puerto Rico [Member] | Commonwealth of Puerto Rico - General Obligation Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,419	1,419
Constitutionally Guaranteed [Member] | Puerto Rico [Member] | Puerto Rico Public Buildings Authority [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	141	141
Public Corporations, Subject To Clawback [Member] | Puerto Rico [Member] | PRHTA (Transportation revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	882	882
Public Corporations, Subject To Clawback [Member] | Puerto Rico [Member] | PRHTA (Highway revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	495	495
Public Corporations, Subject To Clawback [Member] | Puerto Rico [Member] | PRCCDA (Puerto Rico Convention Center District Authority) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	152	152
Public Corporations, Subject To Clawback [Member] | Puerto Rico [Member] | PRIFA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	18	18
Public Corporations [Member] | Puerto Rico [Member] | PREPA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	853	853
Public Corporations [Member] | Puerto Rico [Member] | PRASA (Puerto Rico Aqueduct and Sewer Authority) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	373	373
Public Corporations [Member] | Puerto Rico [Member] | MFA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	360	360
Public Corporations [Member] | Puerto Rico [Member] | COFINA (Puerto Rico Sales Tax Financing Corporation) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	273	272
Public Corporations [Member] | Puerto Rico [Member] | University of Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1	$ 1