Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule of Insured Financial Obligations [Line Items]
Net	$ 248,144	$ 249,153
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	238,132	239,296
Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	8,328	8,083
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	10,012	9,857
BIG
Schedule of Insured Financial Obligations [Line Items]
Net	5,193	5,437
BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	4,224	4,402
BIG | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	91	94
BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	969	1,035
BIG | BIG 1
Schedule of Insured Financial Obligations [Line Items]
Net	2,371	2,410
BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	2,354	2,388
BIG | BIG 1 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	0	0
BIG | BIG 1 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	17	22
BIG | BIG 2
Schedule of Insured Financial Obligations [Line Items]
Net	987	989
BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	926	926
BIG | BIG 2 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	25	27
BIG | BIG 2 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	61	63
BIG | BIG 3
Schedule of Insured Financial Obligations [Line Items]
Net	1,835	2,038
BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	944	1,088
BIG | BIG 3 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	66	67
BIG | BIG 3 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	891	950
United States | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	189,895	190,289
United States | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	1,684	1,774
United States | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	8,643	8,632
United States | BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	3,119	3,271
United States | BIG | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	878	941
United States | BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	969	1,035
United States | BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	1,249	1,257
United States | BIG | BIG 1 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	17	22
United States | BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	926	926
United States | BIG | BIG 2 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	36	36
United States | BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	944	1,088
United States | BIG | BIG 3 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	825	883
Non-U.S. public finance | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	48,237	49,007
Non-U.S. public finance | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	1,369	1,225
Non-U.S. public finance | BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	1,105	1,131
Non-U.S. public finance | BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	0	0
Non-U.S. public finance | BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	1,105	1,131
Non-U.S. public finance | BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	0	0
Non-U.S. public finance | BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 0	$ 0