Outstanding Exposure - Puerto Rico Net Par Outstanding (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 235,668	$ 234,153
Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	3,572	3,725
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	3,391	3,500
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | Commonwealth of Puerto Rico
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,097	1,112
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | PBA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	122	134
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | GO/PBA PSA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,219	1,246
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | PRHTA (Transportation revenue)
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	799	817
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | PRHTA (Highway revenue)
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	457	493
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | PRCCDA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	152	152
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | HTA/CCDA PSA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,408	1,462
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | PREPA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	748	776
Puerto Rico | Puerto Rico Exposures Subject to a Support Agreement | PRIFA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	16	16
Puerto Rico | Other Puerto Rico Exposures
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	181	225
Puerto Rico | Other Puerto Rico Exposures | MFA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	179	223
Puerto Rico | Other Puerto Rico Exposures | PRASA and U of PR
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 2	$ 2