Outstanding Exposure - Puerto Rico Net Par Outstanding (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 229,375	$ 241,802
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	4,270	4,767
Constitutionally Guaranteed [Member] | Puerto Rico [Member] | Commonwealth of Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,253	1,340
Constitutionally Guaranteed [Member] | Puerto Rico [Member] | Puerto Rico Public Buildings Authority [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	140	142
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	811	844
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	454	475
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	16
Other Public Corporations [Member] | Puerto Rico [Member] | PREPA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	822	848
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	248	303
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	0
Other Public Corporations [Member] | Puerto Rico [Member] | University of Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 1	$ 1