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Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 261,552	$ 249,153
% of total net par outstanding	100.00%	100.00%
AAA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 3,081	$ 3,504
% of total net par outstanding	1.20%	1.40%
AA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 25,962	$ 25,868
% of total net par outstanding	9.90%	10.40%
A
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 127,617	$ 118,123
% of total net par outstanding	48.80%	47.40%
BBB
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 94,710	$ 96,221
% of total net par outstanding	36.20%	38.60%
BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 10,182	$ 5,437
% of total net par outstanding	3.90%	2.20%
U.S.
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 209,628
Non U.S.
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	51,924
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	250,375	$ 239,296
Public finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	9,286	4,402
Public finance | U.S.
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 201,175	$ 190,289
% of total net par outstanding	100.00%	100.00%
Public finance | U.S. | AAA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 25	$ 110
% of total net par outstanding	0.00%	0.10%
Public finance | U.S. | AA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 17,664	$ 17,883
% of total net par outstanding	8.80%	9.40%
Public finance | U.S. | A
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 111,502	$ 102,945
% of total net par outstanding	55.50%	54.10%
Public finance | U.S. | BBB
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 69,096	$ 66,080
% of total net par outstanding	34.30%	34.70%
Public finance | U.S. | BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 2,888	$ 3,271
% of total net par outstanding	1.40%	1.70%
Public finance | Non U.S.
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 49,200	$ 49,007
% of total net par outstanding	100.00%	100.00%
Public finance | Non U.S. | AAA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 2,074	$ 2,062
% of total net par outstanding	4.20%	4.20%
Public finance | Non U.S. | AA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 2,854	$ 3,379
% of total net par outstanding	5.80%	6.90%
Public finance | Non U.S. | A
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 13,046	$ 12,968
% of total net par outstanding	26.50%	26.50%
Public finance | Non U.S. | BBB
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 24,828	$ 29,467
% of total net par outstanding	50.50%	60.10%
Public finance | Non U.S. | BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 6,398	$ 1,131
% of total net par outstanding	13.00%	2.30%
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 11,177	$ 9,857
Structured finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	896	1,035
Structured finance | U.S.
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 8,453	$ 8,632
% of total net par outstanding	100.00%	100.00%
Structured finance | U.S. | AAA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 512	$ 867
% of total net par outstanding	6.10%	10.00%
Structured finance | U.S. | AA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 5,386	$ 4,517
% of total net par outstanding	63.70%	52.30%
Structured finance | U.S. | A
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 952	$ 1,639
% of total net par outstanding	11.30%	19.00%
Structured finance | U.S. | BBB
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 707	$ 574
% of total net par outstanding	8.30%	6.70%
Structured finance | U.S. | BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 896	$ 1,035
% of total net par outstanding	10.60%	12.00%
Structured finance | Non U.S.
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 2,724	$ 1,225
% of total net par outstanding	100.00%	100.00%
Structured finance | Non U.S. | AAA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 470	$ 465
% of total net par outstanding	17.30%	38.00%
Structured finance | Non U.S. | AA
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 58	$ 89
% of total net par outstanding	2.10%	7.30%
Structured finance | Non U.S. | A
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 2,117	$ 571
% of total net par outstanding	77.70%	46.60%
Structured finance | Non U.S. | BBB
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 79	$ 100
% of total net par outstanding	2.90%	8.10%
Structured finance | Non U.S. | BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 0	$ 0
% of total net par outstanding	0.00%	0.00%

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