Outstanding Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Sep. 30, 2023	Aug. 31, 2023	Dec. 31, 2022
Schedule of Insured Financial Obligations [Line Items]
Net	$ 241,833	$ 108	$ 233,258
Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	231,721		224,099
Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	8,295		7,203
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	10,112		9,159
BIG
Schedule of Insured Financial Obligations [Line Items]
Net	5,174		5,892
BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	4,122		4,777
BIG | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	96		105
BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	1,052		1,115
BIG | BIG 1
Schedule of Insured Financial Obligations [Line Items]
Net	2,131		3,363
BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	2,108		3,345
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	23		18
BIG | BIG 2
Schedule of Insured Financial Obligations [Line Items]
Net	990		181
BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	926		108
BIG | BIG 2 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	28		34
BIG | BIG 2 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	64		73
BIG | BIG 3
Schedule of Insured Financial Obligations [Line Items]
Net	2,053		2,348
BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	1,088		1,324
BIG | BIG 3 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	68		71
BIG | BIG 3 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	965		1,024
United States | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	185,973		179,636
United States | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	1,817		1,956
United States | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	8,975		8,228
United States | BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	3,304		3,796
United States | BIG | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	956		1,010
United States | BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	1,052		1,115
United States | BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	1,290		2,364
United States | BIG | BIG 1 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	23		18
United States | BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	926		108
United States | BIG | BIG 2 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	36		39
United States | BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	1,088		1,324
United States | BIG | BIG 3 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Net	897		953
Non-U.S. public finance | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	45,748		44,463
Non-U.S. public finance | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Net	1,137		931
Non-U.S. public finance | BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Net	818		981
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	818		981
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