Outstanding Insurance Exposure - Components of BIG Net Par Outstanding (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 233,075	$ 236,807
Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	223,812	226,746
Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	6,131	6,515
Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	9,263	10,061
BIG
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	7,967	8,506
BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	6,211	6,669
BIG | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	232	219
BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,756	1,837
BIG | BIG 1
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,420	2,667
BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	2,130	2,436
BIG | BIG 1 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	94	69
BIG | BIG 1 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	290	231
BIG | BIG 2
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	528	566
BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	429	430
BIG | BIG 2 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	54	62
BIG | BIG 2 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	99	136
BIG | BIG 3
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	5,019	5,273
BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	3,652	3,803
BIG | BIG 3 | Other structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	84	88
BIG | BIG 3 | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,367	1,470
U.S. | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	172,570	175,487
U.S. | RMBS
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	3,132	3,546
U.S. | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	8,581	9,300
U.S. | BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	5,351	5,771
U.S. | BIG | RMBS
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,524	1,618
U.S. | BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,715	1,796
U.S. | BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,318	1,582
U.S. | BIG | BIG 1 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	196	162
U.S. | BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	429	430
U.S. | BIG | BIG 2 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	45	74
U.S. | BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	3,604	3,759
U.S. | BIG | BIG 3 | RMBS
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,283	1,382
Non United States | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	51,242	51,259
Non United States | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	682	761
Non United States | BIG | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	860	898
Non United States | BIG | Structured finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	41	41
Non United States | BIG | BIG 1 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	812	854
Non United States | BIG | BIG 2 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	0	0
Non United States | BIG | BIG 3 | Public finance
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 48	$ 44