Outstanding Exposure - Puerto Rico Net Par Outstanding (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 246,940	$ 264,952
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	4,767	4,966
Constitutionally Guaranteed [Member] | Puerto Rico [Member] | Commonwealth of Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,341	1,419
Constitutionally Guaranteed [Member] | Puerto Rico [Member] | Puerto Rico Public Buildings Authority [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	141	141
Public Corporations, Certain Revenue Potentially Subject to Clawback [Member] | Puerto Rico [Member] | PRHTA (Transportation revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	844	882
Public Corporations, Certain Revenue Potentially Subject to Clawback [Member] | Puerto Rico [Member] | PRHTA (Highway revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	475	495
Public Corporations, Certain Revenue Potentially Subject to Clawback [Member] | Puerto Rico [Member] | PRCCDA (Puerto Rico Convention Center District Authority) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	152	152
Public Corporations, Certain Revenue Potentially Subject to Clawback [Member] | Puerto Rico [Member] | PRIFA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	16	18
Other Public Corporations [Member] | Puerto Rico [Member] | PREPA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	848	853
Other Public Corporations [Member] | Puerto Rico [Member] | PRASA (Puerto Rico Aqueduct and Sewer Authority) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	373	373
Other Public Corporations [Member] | Puerto Rico [Member] | MFA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	303	360
Other Public Corporations [Member] | Puerto Rico [Member] | COFINA (Puerto Rico Sales Tax Financing Corporation) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	273	272
Other Public Corporations [Member] | Puerto Rico [Member] | University of Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1	$ 1