Outstanding Insurance Exposure - Narrative (Details) - USD ($) $ in Millions				1 Months Ended	9 Months Ended
	Jun. 29, 2020	Jun. 26, 2020	Feb. 12, 2019	Jul. 31, 2019	Sep. 30, 2020	Feb. 28, 2020	Feb. 09, 2020	Dec. 31, 2019	May 03, 2019	Jan. 14, 2019
Schedule of Insured Financial Obligations [Line Items]
Second-to-pay insured par outstanding					$ 5,800			$ 6,600
Loss mitigation and other securities					1,400			1,400
Residual value insurance policies exposure downgraded					1,405			1,444
Total net exposure					233,075			236,807
Net exposure					1,108			1,141
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					9,263			10,061
Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					223,812			226,746
Aircraft residual value insurance (RVI)
Schedule of Insured Financial Obligations [Line Items]
Residual value insurance policies exposure downgraded					380			398
Net exposure					$ 225			243
BIG
Schedule of Insured Financial Obligations [Line Items]
Maximum period of liquidity claims (in years)					1 year
Residual value insurance policies exposure downgraded					$ 30
Total net exposure					7,967			8,506
BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					1,756			1,837
BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					6,211			6,669
AAA
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					4,108			4,361
U.S. | RMBS
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					3,132			3,546
U.S. | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					8,581			9,300
U.S. | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					172,570			175,487
U.S. | BIG | RMBS
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					1,524			1,618
U.S. | BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					1,715			1,796
U.S. | BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					5,351			5,771
U.S. | AAA | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					1,068			1,258
U.S. | AAA | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					360			381
Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					4,096			4,270
Puerto Rico | BIG
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					4,100
U.S. Virgin Islands
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					484
U.S. Virgin Islands | BIG
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					218
U.S. Virgin Islands | Internal Investment Grade
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					266
Commitment to Provide Guarantees | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties					573
Commitment to Provide Guarantees | Public finance
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties					$ 375
Minimum
Schedule of Insured Financial Obligations [Line Items]
Probability of paying more claims than being reimbursed (as a percent)					50.00%
Maximum | Capital relieve triple-X life insurance
Schedule of Insured Financial Obligations [Line Items]
Net exposure					$ 900
PRASA (Puerto Rico Aqueduct and Sewer Authority)
Schedule of Insured Financial Obligations [Line Items]
Certified fiscal plan, commonwealth debt service, period	5 years
PRHTA (Highway revenue) | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Obligations secured by taxes on crude oil, unfinished oil and derivative products					120
PRHTA
Schedule of Insured Financial Obligations [Line Items]
Certified fiscal plan, commonwealth debt service, period		5 years
PREPA
Schedule of Insured Financial Obligations [Line Items]
Certified fiscal plan, commonwealth debt service, period	5 years
Constitutionally Guaranteed | Commonwealth of Puerto Rico | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					1,112			1,253
General obligation plan support agreement, outstanding principal amount, net							$ 8,000
Fiscal plan, commonwealth general obligation bonds, litigation amount						$ 35,000	$ 35,000
Constitutionally Guaranteed | Commonwealth of Puerto Rico | General Obligations Bonds Issues On or After March 2012 | Puerto Rico
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 | Commonwealth of Puerto Rico | General Obligations Bonds Issued in or after 2011 | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Fiscal plan, commonwealth surplus available for debt service					210
Constitutionally Guaranteed | Puerto Rico Public Buildings Authority | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					134			140
Public Corporations, Potentially Subject To Clawback | PRHTA (Highway revenue) | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					493			454
Public Corporations, Potentially Subject To Clawback | PRIFA | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					16			16
Public Corporations, Potentially Subject To Clawback | PRHTA | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					817			811
Public Corporations, Potentially Subject To Clawback | Puerto Rico Convention Center District Authority | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					152			152
Other Public Corporations | Puerto Rico Sales Tax Financing Corporation | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			$ 273
Other Public Corporations | PRASA (Puerto Rico Aqueduct and Sewer Authority) | Puerto Rico
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 | MFA | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					223			248
Other Public Corporations | PREPA | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					775			822
Fiscal plan, commonwealth contractual debt service restructuring, minimum bondholder support threshold, percent									67.00%
Other Public Corporations | University of Puerto Rico | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Total net exposure					1			1
Pension Obligations | Commonwealth of Puerto Rico | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Fiscal plan, commonwealth general obligation bonds, litigation amount						$ 50,000
External Credit Rating, Non Investment Grade | Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer
Schedule of Insured Financial Obligations [Line Items]
Second-to-pay insured par outstanding					$ 92			$ 105
Closed Lien Senior Bonds | Puerto Rico Sales Tax Financing Corporation | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Financial guaranty insurance, plan of adjustment, closed lien senior bonds received			$ 152
Subordinated Bonds | Constitutionally Guaranteed | Puerto Rico Sales Tax Financing Corporation | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Expected repayments of debt, percent			60.00%
COFINA Exchange Senior Bonds | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Plan of adjustment, closed lien senior bonds received, fair value			$ 139