Outstanding Exposure - Puerto Rico Net Par Outstanding (Details) - USD ($) $ in Millions			12 Months Ended
	Oct. 23, 2018	Apr. 20, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 20, 2018	Jul. 30, 2018
Schedule of Insured Financial Obligations [Line Items]
Increase (decrease) in net par outstanding			$ (1,457)	$ (5,107)	$ (28)
Net par amount outstanding			241,802	264,952
Commonwealth of Puerto Rico - General Obligation Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Commonwealth debt service, period	6 years
PRHTA (Transportation revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Commonwealth debt service, period		6 years
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			4,767	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			1,340	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			142	141
Constitutionally Guaranteed [Member] | Puerto Rico [Member] | PREPA [Member]
Schedule of Insured Financial Obligations [Line Items]
Expected repayments of debt, percent							$ 3,000
Constitutionally Guaranteed [Member] | Puerto Rico [Member] | COFINA [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal Plan, Commonwealth, Sales and Use Tax, Rate						5.50%
Public Corporations - Certain Revenues Potentially Subject to Clawback [Member] | Puerto Rico [Member] | PRHTA (Transportation revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			844	882
Public Corporations - Certain Revenues Potentially Subject to Clawback [Member] | Puerto Rico [Member] | PRHTA (Highway revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			475	495
Public Corporations - Certain Revenues Potentially Subject to Clawback [Member] | Puerto Rico [Member] | PRCCDA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			152	152
Public Corporations - Certain Revenues Potentially Subject to Clawback [Member] | Puerto Rico [Member] | PRIFA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			16	18
Other Public Corporations [Member] | Puerto Rico [Member] | PREPA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			848	853
Other Public Corporations [Member] | Puerto Rico [Member] | PRASA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			373	373
Other Public Corporations [Member] | Puerto Rico [Member] | MFA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			303	360
Other Public Corporations [Member] | Puerto Rico [Member] | COFINA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			273	272
Other Public Corporations [Member] | Puerto Rico [Member] | U of PR [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			$ 1	$ 1