Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule of Insured Financial Obligations [Line Items]
Net	$ 248,144	$ 249,153
% of total net par outstanding	100.00%	100.00%
AAA
Schedule of Insured Financial Obligations [Line Items]
Net	$ 3,301	$ 3,504
% of total net par outstanding	1.30%	1.40%
AA
Schedule of Insured Financial Obligations [Line Items]
Net	$ 26,883	$ 25,868
% of total net par outstanding	10.80%	10.40%
A
Schedule of Insured Financial Obligations [Line Items]
Net	$ 117,845	$ 118,123
% of total net par outstanding	47.50%	47.40%
BBB
Schedule of Insured Financial Obligations [Line Items]
Net	$ 94,922	$ 96,221
% of total net par outstanding	38.30%	38.60%
BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 5,193	$ 5,437
% of total net par outstanding	2.10%	2.20%
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 238,132	$ 239,296
Public finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 189,895	$ 190,289
% of total net par outstanding	100.00%	100.00%
Public finance | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 48,237	$ 49,007
% of total net par outstanding	100.00%	100.00%
Public finance | AAA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 27	$ 110
% of total net par outstanding	0.00%	0.10%
Public finance | AAA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 2,058	$ 2,062
% of total net par outstanding	4.30%	4.20%
Public finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 17,872	$ 17,883
% of total net par outstanding	9.50%	9.40%
Public finance | AA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 3,336	$ 3,379
% of total net par outstanding	6.90%	6.90%
Public finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 103,551	$ 102,945
% of total net par outstanding	54.50%	54.10%
Public finance | A | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 12,740	$ 12,968
% of total net par outstanding	26.40%	26.50%
Public finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 65,326	$ 66,080
% of total net par outstanding	34.40%	34.70%
Public finance | BBB | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 28,998	$ 29,467
% of total net par outstanding	60.10%	60.10%
Public finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 4,224	$ 4,402
Public finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 3,119	$ 3,271
% of total net par outstanding	1.60%	1.70%
Public finance | BIG | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 1,105	$ 1,131
% of total net par outstanding	2.30%	2.30%
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 10,012	$ 9,857
Structured finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 8,643	$ 8,632
% of total net par outstanding	100.00%	100.00%
Structured finance | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 1,369	$ 1,225
% of total net par outstanding	100.00%	100.00%
Structured finance | AAA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 762	$ 867
% of total net par outstanding	8.80%	10.00%
Structured finance | AAA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 454	$ 465
% of total net par outstanding	33.20%	38.00%
Structured finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 5,591	$ 4,517
% of total net par outstanding	64.70%	52.30%
Structured finance | AA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 84	$ 89
% of total net par outstanding	6.10%	7.30%
Structured finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 816	$ 1,639
% of total net par outstanding	9.50%	19.00%
Structured finance | A | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 738	$ 571
% of total net par outstanding	53.90%	46.60%
Structured finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 505	$ 574
% of total net par outstanding	5.80%	6.70%
Structured finance | BBB | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 93	$ 100
% of total net par outstanding	6.80%	8.10%
Structured finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 969	$ 1,035
Structured finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 969	$ 1,035
% of total net par outstanding	11.20%	12.00%
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%