Outstanding Exposure - Puerto Rico Net Par Outstanding (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) judge	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Increase (decrease) in net par outstanding	$ (5,107)	$ (28)	$ (855)
Net par amount outstanding	$ 264,952	296,318
Number of Federal Judges | judge	5
PREPA [Member]
Schedule of Insured Financial Obligations [Line Items]
Commonwealth debt service, period	6 years
Expected repayments of debt, percent	47.00%
PRASA [Member]
Schedule of Insured Financial Obligations [Line Items]
Commonwealth debt service, period	5 years
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 4,966	4,786
Puerto Rico [Member] | Commonwealth of Puerto Rico - General Obligation Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Increase (decrease) in net par outstanding	36
Puerto Rico [Member] | PRHTA (Highway revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Increase (decrease) in net par outstanding	144
Puerto Rico [Member] | PREPA [Member]
Schedule of Insured Financial Obligations [Line Items]
Increase (decrease) in net par outstanding	134
Puerto Rico [Member] | MFA [Member]
Schedule of Insured Financial Obligations [Line Items]
Increase (decrease) in net par outstanding	75
Puerto Rico [Member] | Commutation of Previously Ceded Business [Member]
Schedule of Insured Financial Obligations [Line Items]
Increase (decrease) in net par outstanding	389
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	1,476
Constitutionally Guaranteed [Member] | Puerto Rico [Member] | Puerto Rico Public Buildings Authority [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	141	169
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	882	918
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	495	350
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	18	18
Other Public Corporations [Member] | Puerto Rico [Member] | PREPA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	853	724
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	360	334
Other Public Corporations [Member] | Puerto Rico [Member] | COFINA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	272	271
Other Public Corporations [Member] | Puerto Rico [Member] | U of PR [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1	$ 1