Outstanding Exposure - Financial Guaranty Portfolio by Internal Rating (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Schedule of Insured Financial Obligations [Line Items]
Net	$ 227,071	$ 236,392
% of total net par outstanding	100.00%	100.00%
AAA
Schedule of Insured Financial Obligations [Line Items]
Net	$ 3,364	$ 3,788
% of total net par outstanding	1.50%	1.60%
AA
Schedule of Insured Financial Obligations [Line Items]
Net	$ 24,438	$ 25,359
% of total net par outstanding	10.80%	10.70%
A
Schedule of Insured Financial Obligations [Line Items]
Net	$ 105,223	$ 106,091
% of total net par outstanding	46.30%	44.90%
BBB
Schedule of Insured Financial Obligations [Line Items]
Net	$ 88,261	$ 93,798
% of total net par outstanding	38.90%	39.70%
BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 5,785	$ 7,356
% of total net par outstanding	2.50%	3.10%
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 218,905	$ 227,164
Public finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 177,842	$ 177,219
% of total net par outstanding	100.00%	100.00%
Public finance | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 41,063	$ 49,945
% of total net par outstanding	100.00%	100.00%
Public finance | AAA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 259	$ 272
% of total net par outstanding	0.10%	0.20%
Public finance | AAA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 1,879	$ 2,217
% of total net par outstanding	4.60%	4.50%
Public finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 16,451	$ 16,372
% of total net par outstanding	9.30%	9.20%
Public finance | AA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 3,454	$ 4,205
% of total net par outstanding	8.40%	8.40%
Public finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 96,052	$ 94,459
% of total net par outstanding	54.00%	53.30%
Public finance | A | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 8,491	$ 10,659
% of total net par outstanding	20.70%	21.30%
Public finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 60,893	$ 60,744
% of total net par outstanding	34.20%	34.30%
Public finance | BBB | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 26,777	$ 32,264
% of total net par outstanding	65.20%	64.60%
Public finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 4,649	$ 5,972
Public finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 4,187	$ 5,372
% of total net par outstanding	2.40%	3.00%
Public finance | BIG | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 462	$ 600
% of total net par outstanding	1.10%	1.20%
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 8,166	$ 9,228
Structured finance | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 7,449	$ 8,374
% of total net par outstanding	100.00%	100.00%
Structured finance | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 717	$ 854
% of total net par outstanding	100.00%	100.00%
Structured finance | AAA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 794	$ 806
% of total net par outstanding	10.70%	9.60%
Structured finance | AAA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 432	$ 493
% of total net par outstanding	60.30%	57.70%
Structured finance | AA | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 4,522	$ 4,760
% of total net par outstanding	60.70%	56.80%
Structured finance | AA | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 11	$ 22
% of total net par outstanding	1.50%	2.60%
Structured finance | A | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 510	$ 813
% of total net par outstanding	6.80%	9.70%
Structured finance | A | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 170	$ 160
% of total net par outstanding	23.70%	18.70%
Structured finance | BBB | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 487	$ 611
% of total net par outstanding	6.50%	7.30%
Structured finance | BBB | Non-U.S. public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 104	$ 179
% of total net par outstanding	14.50%	21.00%
Structured finance | BIG
Schedule of Insured Financial Obligations [Line Items]
Net	$ 1,136	$ 1,384
Structured finance | BIG | United States
Schedule of Insured Financial Obligations [Line Items]
Net	$ 1,136	$ 1,384
% of total net par outstanding	15.30%	16.60%
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%