Outstanding Exposure - Narrative (Details) - USD ($) $ in Millions				9 Months Ended
	Jun. 05, 2019	May 09, 2019	Feb. 12, 2019	Sep. 30, 2019	Sep. 27, 2019	Jun. 27, 2019	Jun. 16, 2019	Feb. 19, 2019	Jan. 14, 2019	Dec. 31, 2018
Schedule of Insured Financial Obligations [Line Items]
Second-to-pay insured par outstanding				$ 6,400						$ 6,700
Loss mitigation securities				1,500						1,900
Total net exposure				229,375						241,802
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				9,978						11,137
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				219,397						230,665
Aircraft residual value insurance (RVI) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Exposure				$ 243						218
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Maximum period of liquidity claims (in years)				1 year
Total net exposure				$ 8,515						10,160
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				1,892						2,731
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				6,623						7,429
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				4,337						4,618
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				3,687						4,270
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				9,226						9,944
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				176,515						186,562
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				1,666						2,387
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				1,851						2,632
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				5,760						6,388
United States [Member] | AAA [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				1,345						1,533
United States [Member] | AAA [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				384						413
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				4,270						4,767
Puerto Rico [Member] | COFINA Exchange Senior Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Plan of Adjustment, closed lien senior bonds received, initial par value			$ 152
Plan of adjustment closed lien senior bonds received, fair value			$ 139
Puerto Rico [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				4,300
U.S. Virgin Islands [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				494
U.S. Virgin Islands [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				220
U.S. Virgin Islands [Member] | Internal Investment Grade [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				274
Commitment to Provide Guarantees [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties				500
Commitment to Provide Guarantees [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties				$ 983
Minimum [Member]
Schedule of Insured Financial Obligations [Line Items]
Probability of paying more claims than being reimbursed (as a percent)				50.00%
Maximum [Member] | Capital relieve triple-X life insurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Exposure				$ 1,000
Commonwealth of Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Certified fiscal plan, commonwealth debt service, period		6 years
PRHTA (Highway revenue) [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Obligations secured by taxes on crude oil, unfinished oil and derivative products				120
PRHTA [Member]
Schedule of Insured Financial Obligations [Line Items]
Certified fiscal plan, commonwealth debt service, period	6 years
Constitutionally Guaranteed [Member] | Commonwealth of Puerto Rico [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				1,253						1,340
Fiscal plan, commonwealth general obligation bonds, litigation amount					$ 35,000		$ 35,000
Vintage GO bondholders, expected receivable of outstanding claims, percent							64.30%
Vintage GO bondholders, additional expected receivable of outstanding claims, percent							25.10%
Expected receivable of outstanding claims, additional amount from Commonwealth if it outperforms fiscal plan, percent							16.80%
Fiscal plan, commonwealth surplus available for debt service		$ 13,700
General Obligation Plan Support Agreement, Outstanding Principal Amount, Net							$ 3,000
Constitutionally Guaranteed [Member] | Commonwealth of Puerto Rico [Member] | General Obligations Bonds Issues On or After March 2012 [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal plan, commonwealth general obligation bonds, litigation amount									$ 6,000
Fiscal plan, commonwealth surplus available for debt service				369
Constitutionally Guaranteed [Member] | Commonwealth of Puerto Rico [Member] | General Obligations Bonds Issued in or after 2011 [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal plan, commonwealth surplus available for debt service				215
Constitutionally Guaranteed [Member] | Commonwealth of Puerto Rico [Member] | Maximum [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Vintage GO bondholders, additional expected receivable of outstanding claims, percent							89.40%
Constitutionally Guaranteed [Member] | COFINA (Puerto Rico Sales Tax Financing Corporation) [Member] | Puerto Rico [Member] | Subordinated Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Expected repayments of debt, percent			60.00%
Constitutionally Guaranteed [Member] | Puerto Rico Public Buildings Authority [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				140						142
Vintage GO bondholders, expected receivable of outstanding claims, percent							72.60%
Constitutionally Guaranteed [Member] | Puerto Rico Public Buildings Authority [Member] | Maximum [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Vintage GO bondholders, additional expected receivable of outstanding claims, percent							89.40%
Other Public Corporations [Member] | PRASA (Puerto Rico Aqueduct and Sewer Authority) [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				$ 373						373
Minimum period in forbearance				3 years
Debt instrument, term				40 years
Non-interest accrual period				10 years
Other Public Corporations [Member] | MFA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				$ 248						303
Other Public Corporations [Member] | COFINA (Puerto Rico Sales Tax Financing Corporation) [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				0
Other Public Corporations [Member] | COFINA (Puerto Rico Sales Tax Financing Corporation) [Member] | Puerto Rico [Member] | Closed Lien Senior Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure								$ 273
Other Public Corporations [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				822						848
Fiscal plan, commonwealth contractual debt service restructuring, minimum bondholder support threshold, percent						67.00%
Pension Obligations [Member] | Commonwealth of Puerto Rico [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal plan, commonwealth general obligation bonds, litigation amount					$ 50,000
External Credit Rating, Non Investment Grade [Member] | Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member]
Schedule of Insured Financial Obligations [Line Items]
Second-to-pay insured par outstanding				$ 106						$ 111