Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Schedule of Insured Financial Obligations [Line Items]
Net	$ 263,591	$ 261,552
% of total net par outstanding	100.00%	100.00%
AAA
Schedule of Insured Financial Obligations [Line Items]
Net	$ 3,064	$ 3,081
% of total net par outstanding	1.20%	1.20%
AA
Schedule of Insured Financial Obligations [Line Items]
Net	$ 25,789	$ 25,962
% of total net par outstanding	9.80%	9.90%
A
Schedule of Insured Financial Obligations [Line Items]
Net	$ 128,573	$ 127,617
% of total net par outstanding	48.80%	48.80%
BBB
Schedule of Insured Financial Obligations [Line Items]
Net	$ 96,623	$ 94,710
% of total net par outstanding	36.60%	36.20%
BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 9,542	$ 10,182
% of total net par outstanding	3.60%	3.90%
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 252,531	$ 250,375
Public finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 202,417	$ 201,175
% of total net par outstanding	100.00%	100.00%
Public finance | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 50,114	$ 49,200
% of total net par outstanding	100.00%	100.00%
Public finance | AAA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 24	$ 25
% of total net par outstanding	0.00%	0.00%
Public finance | AAA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 2,050	$ 2,074
% of total net par outstanding	4.10%	4.20%
Public finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 17,579	$ 17,664
% of total net par outstanding	8.70%	8.80%
Public finance | AA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 2,906	$ 2,854
% of total net par outstanding	5.80%	5.80%
Public finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 113,268	$ 111,502
% of total net par outstanding	56.00%	55.50%
Public finance | A | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 12,226	$ 13,046
% of total net par outstanding	24.40%	26.50%
Public finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 69,500	$ 69,096
% of total net par outstanding	34.30%	34.30%
Public finance | BBB | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 26,317	$ 24,828
% of total net par outstanding	52.50%	50.50%
Public finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 8,661	$ 9,286
Public finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 2,046	$ 2,888
% of total net par outstanding	1.00%	1.40%
Public finance | BIG | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 6,615	$ 6,398
% of total net par outstanding	13.20%	13.00%
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 11,060	$ 11,177
Structured finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 8,373	$ 8,453
% of total net par outstanding	100.00%	100.00%
Structured finance | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 2,687	$ 2,724
% of total net par outstanding	100.00%	100.00%
Structured finance | AAA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 503	$ 512
% of total net par outstanding	6.00%	6.10%
Structured finance | AAA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 487	$ 470
% of total net par outstanding	18.10%	17.30%
Structured finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 5,242	$ 5,386
% of total net par outstanding	62.60%	63.70%
Structured finance | AA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 62	$ 58
% of total net par outstanding	2.30%	2.10%
Structured finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 1,019	$ 952
% of total net par outstanding	12.20%	11.30%
Structured finance | A | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 2,060	$ 2,117
% of total net par outstanding	76.70%	77.70%
Structured finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 728	$ 707
% of total net par outstanding	8.70%	8.30%
Structured finance | BBB | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 78	$ 79
% of total net par outstanding	2.90%	2.90%
Structured finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 881	$ 896
Structured finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 881	$ 896
% of total net par outstanding	10.50%	10.60%
Structured finance | BIG | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 0	$ 0
% of total net par outstanding	0.00%	0.00%