Outstanding Exposure - Geographic Distribution of Net Par Outstanding (Details) $ in Millions	Dec. 31, 2023 USD ($) risk	Aug. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	7,754
Total net par outstanding	$ 249,153	$ 108	$ 233,258
Percent of Total Net Par Outstanding	100.00%
U.S.
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	7,415
Total net par outstanding	$ 198,921
Percent of Total Net Par Outstanding	79.80%
Non U.S.
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	339
Total net par outstanding	$ 50,232
Percent of Total Net Par Outstanding	20.20%
U.K.
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	271
Total net par outstanding	$ 39,394
Percent of Total Net Par Outstanding	15.80%
Canada
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	5
Total net par outstanding	$ 1,696
Percent of Total Net Par Outstanding	0.70%
Spain
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	9
Total net par outstanding	$ 1,649
Percent of Total Net Par Outstanding	0.70%
France
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	7
Total net par outstanding	$ 1,565
Percent of Total Net Par Outstanding	0.60%
Australia
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	6
Total net par outstanding	$ 1,518
Percent of Total Net Par Outstanding	0.60%
Other
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	41
Total net par outstanding	$ 4,410
Percent of Total Net Par Outstanding	1.80%
Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net par outstanding	$ 239,296		224,099
Public finance | U.S.
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	7,056
Total net par outstanding	$ 190,289		179,636
Percent of Total Net Par Outstanding	76.30%
Public finance | California
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	1,213
Total net par outstanding	$ 36,200
Percent of Total Net Par Outstanding	14.50%
Public finance | Texas
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	1,077
Total net par outstanding	$ 22,783
Percent of Total Net Par Outstanding	9.10%
Public finance | New York
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	774
Total net par outstanding	$ 17,751
Percent of Total Net Par Outstanding	7.10%
Public finance | Pennsylvania
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	534
Total net par outstanding	$ 16,941
Percent of Total Net Par Outstanding	6.80%
Public finance | Illinois
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	485
Total net par outstanding	$ 12,953
Percent of Total Net Par Outstanding	5.20%
Public finance | New Jersey
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	256
Total net par outstanding	$ 9,255
Percent of Total Net Par Outstanding	3.70%
Public finance | Florida
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	214
Total net par outstanding	$ 8,586
Percent of Total Net Par Outstanding	3.50%
Public finance | Michigan
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	231
Total net par outstanding	$ 5,533
Percent of Total Net Par Outstanding	2.20%
Public finance | Louisiana
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	132
Total net par outstanding	$ 4,758
Percent of Total Net Par Outstanding	1.90%
Public finance | Alabama
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	243
Total net par outstanding	$ 3,819
Percent of Total Net Par Outstanding	1.50%
Public finance | Other
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	1,897
Total net par outstanding	$ 51,710
Percent of Total Net Par Outstanding	20.80%
Public finance | Non U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net par outstanding	$ 49,007		44,463
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net par outstanding	$ 9,857		9,159
Structured finance | U.S.
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	359
Total net par outstanding	$ 8,632		8,228
Percent of Total Net Par Outstanding	3.50%
Structured finance | Non U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net par outstanding	$ 1,225		$ 931