Outstanding Insurance Exposure - BIG Net Par Outstanding (Details) $ in Millions	Dec. 31, 2019 USD ($) risk	Dec. 31, 2018 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Credit Derivative	$ 5,956	$ 6,182
Net Par Outstanding, Total	$ 236,807	241,802
Number of Risks | risk	8,849
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 8,377	10,003
Net Par Outstanding, Credit Derivative	129	157
Net Par Outstanding, Total	$ 8,506	$ 10,160
Number of Risks, Financial Guaranty Insurance | risk	276	312
Number of Risks, Credit Derivative | risk	14	15
Number of Risks | risk	290	327
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 2,600	$ 2,981
Net Par Outstanding, Credit Derivative	67	77
Net Par Outstanding, Total	$ 2,667	$ 3,058
Number of Risks, Financial Guaranty Insurance | risk	121	128
Number of Risks, Credit Derivative | risk	6	6
Number of Risks | risk	127	134
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 561	$ 932
Net Par Outstanding, Credit Derivative	5	5
Net Par Outstanding, Total	$ 566	$ 937
Number of Risks, Financial Guaranty Insurance | risk	24	39
Number of Risks, Credit Derivative | risk	1	1
Number of Risks | risk	25	40
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 5,216	$ 6,090
Net Par Outstanding, Credit Derivative	57	75
Net Par Outstanding, Total	$ 5,273	$ 6,165
Number of Risks, Financial Guaranty Insurance | risk	131	145
Number of Risks, Credit Derivative | risk	7	8
Number of Risks | risk	138	153