Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Sep. 30, 2023	Aug. 31, 2023	Dec. 31, 2022
Schedule of Insured Financial Obligations [Line Items]
Net	$ 241,833	$ 108	$ 233,258
% of total net par outstanding	100.00%		100.00%
AAA
Schedule of Insured Financial Obligations [Line Items]
Net	$ 3,533		$ 3,584
% of total net par outstanding	1.50%		1.50%
AA
Schedule of Insured Financial Obligations [Line Items]
Net	$ 24,849		$ 24,383
% of total net par outstanding	10.30%		10.50%
A
Schedule of Insured Financial Obligations [Line Items]
Net	$ 114,288		$ 107,493
% of total net par outstanding	47.20%		46.10%
BBB
Schedule of Insured Financial Obligations [Line Items]
Net	$ 93,989		$ 91,906
% of total net par outstanding	38.90%		39.40%
BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 5,174		$ 5,892
% of total net par outstanding	2.10%		2.50%
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 231,721		$ 224,099
Public finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 185,973		$ 179,636
% of total net par outstanding	100.00%		100.00%
Public finance | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 45,748		$ 44,463
% of total net par outstanding	100.00%		100.00%
Public finance | AAA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 212		$ 222
% of total net par outstanding	0.10%		0.10%
Public finance | AAA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 2,004		$ 1,967
% of total net par outstanding	4.40%		4.40%
Public finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 16,994		$ 16,241
% of total net par outstanding	9.10%		9.10%
Public finance | AA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 3,298		$ 3,497
% of total net par outstanding	7.20%		7.90%
Public finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 101,243		$ 96,807
% of total net par outstanding	54.50%		53.90%
Public finance | A | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 10,545		$ 9,271
% of total net par outstanding	23.10%		20.90%
Public finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 64,220		$ 62,570
% of total net par outstanding	34.50%		34.80%
Public finance | BBB | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 29,083		$ 28,747
% of total net par outstanding	63.50%		64.60%
Public finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 4,122		$ 4,777
Public finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 3,304		$ 3,796
% of total net par outstanding	1.80%		2.10%
Public finance | BIG | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 818		$ 981
% of total net par outstanding	1.80%		2.20%
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 10,112		$ 9,159
Structured finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 8,975		$ 8,228
% of total net par outstanding	100.00%		100.00%
Structured finance | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 1,137		$ 931
% of total net par outstanding	100.00%		100.00%
Structured finance | AAA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 860		$ 926
% of total net par outstanding	9.60%		11.20%
Structured finance | AAA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 457		$ 469
% of total net par outstanding	40.20%		50.40%
Structured finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 4,545		$ 4,633
% of total net par outstanding	50.70%		56.30%
Structured finance | AA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 12		$ 12
% of total net par outstanding	1.00%		1.30%
Structured finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 1,932		$ 1,075
% of total net par outstanding	21.50%		13.10%
Structured finance | A | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 568		$ 340
% of total net par outstanding	50.00%		36.50%
Structured finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 586		$ 479
% of total net par outstanding	6.50%		5.80%
Structured finance | BBB | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 100		$ 110
% of total net par outstanding	8.80%		11.80%
Structured finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 1,052		$ 1,115
Structured finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 1,052		$ 1,115
% of total net par outstanding	11.70%		13.60%
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%