Outstanding Exposure - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	Aug. 31, 2021	Dec. 31, 2020	Sep. 30, 2021	Nov. 04, 2021	Jul. 30, 2021	Feb. 22, 2021
Schedule of Insured Financial Obligations [Line Items]
Loss mitigation securities and other		$ 1,400	$ 1,300
Net par outstanding		234,153	235,668
Residual value insurance policies exposure downgraded		1,484	1,577
HTA/CCDA PSA | PRHTA (Transportation revenue)
Schedule of Insured Financial Obligations [Line Items]
Cash received for bonds			389
Toll bonds			1,245
HTA/CCDA PSA | Puerto Rico Convention Center District Authority
Schedule of Insured Financial Obligations [Line Items]
Cash received for bonds			112
Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		3,725	3,572
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		3,500	$ 3,391
Percent of total net par outstanding			95.00%
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | GO/PBA PSA
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		1,246	$ 1,219
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | Commonwealth of Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		1,112	1,097
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | Puerto Rico Public Buildings Authority
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		134	122
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | HTA/CCDA PSA
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		1,462	1,408
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | PRHTA (Transportation revenue)
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		817	799
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | PRHTA (Highway revenue)
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		493	457
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | Puerto Rico Convention Center District Authority
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		152	152
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | PREPA
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		776	748
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | PRIFA
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		16	16
Cash received for bonds			204
Puerto Rico | Constitutionally Guaranteed | GO/PBA PSA
Schedule of Insured Financial Obligations [Line Items]
General obligation plan support agreement						$ 11,700
General obligation plan support agreement, percent of aggregate amount						62.00%
Puerto Rico | Constitutionally Guaranteed | Commonwealth of Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Fiscal plan, commonwealth general obligation bonds, litigation amount					$ 35,000
Puerto Rico | Constitutionally Guaranteed | GO and PBA POA
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding			1,116
Amount of outstanding principal to be paid, in percent	100.00%
Net par outstanding of remaining insured securities			102
Puerto Rico | Pension Obligations | Commonwealth of Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Fiscal plan, commonwealth general obligation bonds, litigation amount					$ 50,000
Puerto Rico | HTA/CCDA PSA
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding			1,408
Puerto Rico | Other Public Corporations | University of Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding			1
Puerto Rico | Other Public Corporations | PRASA (Puerto Rico Aqueduct and Sewer Authority)
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding			1
Insured financial obligations, refunded during period		372
U.S. Virgin Islands
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding			477
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		224,625	226,257
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		9,528	9,411
Commitment to Provide Guarantees | Public finance
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties			1,396
Commitment to Provide Guarantees | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties			$ 763
Subsequent Event
Schedule of Insured Financial Obligations [Line Items]
First-time insurance claims paid				$ 10
BIG
Schedule of Insured Financial Obligations [Line Items]
Maximum period of liquidity claims (in years)			1 year
Net par outstanding		7,975	$ 7,346
Residual value insurance policies exposure downgraded		13	84
BIG | Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding			3,600
BIG | U.S. Virgin Islands
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		216	473
BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		6,334	5,920
BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding		$ 1,641	$ 1,426
Minimum
Schedule of Insured Financial Obligations [Line Items]
Probability of paying more claims than being reimbursed (as a percent)			50.00%