Outstanding Exposure - Geographic Distribution of Net Par Outstanding (Details) $ in Millions	Dec. 31, 2024 USD ($) risk	Dec. 31, 2023 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	7,852
Total net par outstanding	$ 261,552	$ 249,153
Percent of Total Net Par Outstanding	100.00%
U.S.
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	7,503
Total net par outstanding	$ 209,628
Percent of Total Net Par Outstanding	80.10%
Non U.S.
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	349
Total net par outstanding	$ 51,924
Percent of Total Net Par Outstanding	19.90%
U.K.
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	272
Total net par outstanding	$ 41,001
Percent of Total Net Par Outstanding	15.70%
Australia
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	7
Total net par outstanding	$ 1,740
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	$ 1,506
Percent of Total Net Par Outstanding	0.60%
France
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	7
Total net par outstanding	$ 1,477
Percent of Total Net Par Outstanding	0.50%
Canada
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	5
Total net par outstanding	$ 1,243
Percent of Total Net Par Outstanding	0.50%
Other
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	49
Total net par outstanding	$ 4,957
Percent of Total Net Par Outstanding	1.90%
Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net par outstanding	$ 250,375	239,296
Public finance | U.S.
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	7,175
Total net par outstanding	$ 201,175	190,289
Percent of Total Net Par Outstanding	76.90%
Public finance | California
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	1,182
Total net par outstanding	$ 36,080
Percent of Total Net Par Outstanding	13.80%
Public finance | Texas
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	1,145
Total net par outstanding	$ 26,004
Percent of Total Net Par Outstanding	9.90%
Public finance | New York
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	798
Total net par outstanding	$ 19,572
Percent of Total Net Par Outstanding	7.50%
Public finance | Pennsylvania
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	533
Total net par outstanding	$ 18,448
Percent of Total Net Par Outstanding	7.10%
Public finance | Illinois
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	479
Total net par outstanding	$ 12,536
Percent of Total Net Par Outstanding	4.80%
Public finance | Florida
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	218
Total net par outstanding	$ 11,353
Percent of Total Net Par Outstanding	4.30%
Public finance | New Jersey
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	248
Total net par outstanding	$ 8,824
Percent of Total Net Par Outstanding	3.40%
Public finance | Louisiana
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	139
Total net par outstanding	$ 4,994
Percent of Total Net Par Outstanding	1.90%
Public finance | Michigan
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	228
Total net par outstanding	$ 4,877
Percent of Total Net Par Outstanding	1.90%
Public finance | Colorado
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	171
Total net par outstanding	$ 4,012
Percent of Total Net Par Outstanding	1.50%
Public finance | Other
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	2,034
Total net par outstanding	$ 54,475
Percent of Total Net Par Outstanding	20.80%
Public finance | Non U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net par outstanding	$ 49,200	49,007
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net par outstanding	$ 11,177	9,857
Structured finance | U.S.
Schedule of Insured Financial Obligations [Line Items]
Number of Risks | risk	328
Total net par outstanding	$ 8,453	8,632
Percent of Total Net Par Outstanding	3.20%
Structured finance | Non U.S.
Schedule of Insured Financial Obligations [Line Items]
Total net par outstanding	$ 2,724	$ 1,225