Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 236,392	$ 234,153
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	227,164	224,625
Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	6,837	6,538
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	9,228	9,528
U.S.
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	185,593
U.S. | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	177,219	171,597
U.S. | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	2,391	2,990
U.S. | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	8,374	8,952
Non U.S.
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	50,799
Non U.S. | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	49,945	53,028
Non U.S. | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	854	576
BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	7,356	7,975
BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	5,972	6,334
BIG | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	119	161
BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,384	1,641
BIG | U.S. | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	5,372	5,439
BIG | U.S. | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,265	1,480
BIG | U.S. | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,384	1,641
BIG | Non U.S. | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	600	895
BIG | Non U.S. | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	0	0
BIG | BIG 1
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	2,443	2,851
BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	2,321	2,623
BIG | BIG 1 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1	28
BIG | BIG 1 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	122	228
BIG | BIG 1 | U.S. | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,765	1,777
BIG | BIG 1 | U.S. | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	121	200
BIG | BIG 1 | Non U.S. | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	556	846
BIG | BIG 2
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	181	134
BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	116	57
BIG | BIG 2 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	41	51
BIG | BIG 2 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	65	77
BIG | BIG 2 | U.S. | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	116	57
BIG | BIG 2 | U.S. | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	24	26
BIG | BIG 2 | Non U.S. | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	0	0
BIG | BIG 3
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	4,732	4,990
BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	3,535	3,654
BIG | BIG 3 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	77	82
BIG | BIG 3 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,197	1,336
BIG | BIG 3 | U.S. | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	3,491	3,605
BIG | BIG 3 | U.S. | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,120	1,254
BIG | BIG 3 | Non U.S. | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 44	$ 49