Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Schedule of Insured Financial Obligations [Line Items]
Net	$ 263,591	$ 261,552
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	252,531	250,375
Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	9,586	9,670
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	11,060	11,177
BIG
Schedule of Insured Financial Obligations [Line Items]
Net	9,542	10,182
BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	8,661	9,286
BIG | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	76	77
BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	881	896
BIG | BIG 1
Schedule of Insured Financial Obligations [Line Items]
Net	7,079	8,102
BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	6,980	7,998
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	99	104
BIG | BIG 2
Schedule of Insured Financial Obligations [Line Items]
Net	1,099	706
BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	1,050	656
BIG | BIG 2 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	20	21
BIG | BIG 2 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	49	50
BIG | BIG 3
Schedule of Insured Financial Obligations [Line Items]
Net	1,364	1,374
BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	631	632
BIG | BIG 3 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	56	56
BIG | BIG 3 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	733	742
United States | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	202,417	201,175
United States | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	1,474	1,507
United States | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	8,373	8,453
United States | BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	2,046	2,888
United States | BIG | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	805	819
United States | BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	881	896
United States | BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	904	2,119
United States | BIG | BIG 1 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	99	104
United States | BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	511	137
United States | BIG | BIG 2 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	29	29
United States | BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	631	632
United States | BIG | BIG 3 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	677	686
Non-U.S. public finance | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	50,114	49,200
Non-U.S. public finance | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	2,687	2,724
Non-U.S. public finance | BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	6,615	6,398
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	6,076	5,879
Non-U.S. public finance | BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	539	519
Non-U.S. public finance | BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	$ 0	$ 0