Outstanding Exposure - Puerto Rico Net Par Outstanding (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 298,243	$ 296,318
Net par due to commutations	1,000
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	4,937	4,786
Net par due to commutations	150
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	1,495	1,476
Constitutionally Guaranteed [Member] | Puerto Rico [Member] | Puerto Rico Public Buildings Authority [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	169	169
Public Corporations, Subject To Clawback [Member] | Puerto Rico [Member] | PRHTA (Transportation revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	918	918
Public Corporations, Subject To Clawback [Member] | Puerto Rico [Member] | PRHTA (Highway revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	409	350
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	777	724
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	354	334
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	271	271
Public Corporations [Member] | Puerto Rico [Member] | University of Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1	$ 1