Outstanding Insurance Exposure - BIG Net Par Outstanding (Details) $ in Millions	Sep. 30, 2020 USD ($) risk rISK	Dec. 31, 2019 USD ($) risk rISK
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Credit Derivative	$ 5,658	$ 5,956
Total	233,075	236,807
BIG
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	7,845	8,377
Net Par Outstanding, Credit Derivative	122	129
Total	$ 7,967	$ 8,506
Number of Risks, Financial Guaranty Insurance | risk	257	276
Number of Risks, Credit Derivative | rISK	14	14
Number of Risks | rISK	271	290
BIG | BIG 1
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 2,347	$ 2,600
Net Par Outstanding, Credit Derivative	73	67
Total	$ 2,420	$ 2,667
Number of Risks, Financial Guaranty Insurance | risk	112	121
Number of Risks, Credit Derivative | rISK	7	6
Number of Risks | rISK	119	127
BIG | BIG 2
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 524	$ 561
Net Par Outstanding, Credit Derivative	4	5
Total	$ 528	$ 566
Number of Risks, Financial Guaranty Insurance | risk	19	24
Number of Risks, Credit Derivative | rISK	1	1
Number of Risks | rISK	20	25
BIG | BIG 3
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 4,974	$ 5,216
Net Par Outstanding, Credit Derivative	45	57
Total	$ 5,019	$ 5,273
Number of Risks, Financial Guaranty Insurance | risk	126	131
Number of Risks, Credit Derivative | rISK	6	7
Number of Risks | rISK	132	138