Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 235,668	$ 234,153
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	226,257	224,625
Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	6,901	6,538
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	9,411	9,528
BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	7,346	7,975
BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	5,920	6,334
BIG | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	124	161
BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,426	1,641
BIG | BIG 1
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	2,397	2,851
BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	2,268	2,623
BIG | BIG 1 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	2	28
BIG | BIG 1 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	129	228
BIG | BIG 2
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	184	134
BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	115	57
BIG | BIG 2 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	44	51
BIG | BIG 2 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	69	77
BIG | BIG 3
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	4,765	4,990
BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	3,537	3,654
BIG | BIG 3 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	78	82
BIG | BIG 3 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,228	1,336
United States | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	175,952	171,597
United States | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	2,510	2,990
United States | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	8,677	8,952
United States | BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	5,443	5,439
United States | BIG | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,302	1,480
United States | BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,426	1,641
United States | BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,837	1,777
United States | BIG | BIG 1 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	127	200
United States | BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	115	57
United States | BIG | BIG 2 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	25	26
United States | BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	3,491	3,605
United States | BIG | BIG 3 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,150	1,254
Non United States | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	50,305	53,028
Non United States | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	734	576
Non United States | BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	477	895
Non United States | BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	0	0
Non United States | BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	431	846
Non United States | BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	0	0
Non United States | BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 46	$ 49