Outstanding Insurance Exposure - BIG Net Par Outstanding (Details) $ in Millions	Mar. 31, 2020 USD ($) risk	Dec. 31, 2019 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Credit Derivative	$ 5,806	$ 5,956
Total	230,898	236,807
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	8,112	8,377
Net Par Outstanding, Credit Derivative	126	129
Total	$ 8,238	$ 8,506
Number of Risks, Financial Guaranty Insurance | risk	259	276
Number of Risks, Credit Derivative | risk	14	14
Number of Risks | risk	273	290
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 2,380	$ 2,600
Net Par Outstanding, Credit Derivative	89	67
Total	$ 2,469	$ 2,667
Number of Risks, Financial Guaranty Insurance | risk	109	121
Number of Risks, Credit Derivative | risk	7	6
Number of Risks | risk	116	127
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 549	$ 561
Net Par Outstanding, Credit Derivative	4	5
Total	$ 553	$ 566
Number of Risks, Financial Guaranty Insurance | risk	22	24
Number of Risks, Credit Derivative | risk	1	1
Number of Risks | risk	23	25
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 5,183	$ 5,216
Net Par Outstanding, Credit Derivative	33	57
Total	$ 5,216	$ 5,273
Number of Risks, Financial Guaranty Insurance | risk	128	131
Number of Risks, Credit Derivative | risk	6	7
Number of Risks | risk	134	138