Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Jun. 30, 2019 USD ($) risk	Dec. 31, 2018 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Credit Derivative	$ 6,160	$ 6,182
Total	235,367	241,802
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	8,715	10,003
Net Par Outstanding, Credit Derivative	148	157
Total	$ 8,863	$ 10,160
Number of Risks, Financial Guaranty Insurance | risk	278	312
Number of Risks, Credit Derivative | risk	15	15
Number of Risks | risk	293	327
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 2,705	$ 2,981
Net Par Outstanding, Credit Derivative	76	77
Total	$ 2,781	$ 3,058
Number of Risks, Financial Guaranty Insurance | risk	116	128
Number of Risks, Credit Derivative | risk	6	6
Number of Risks | risk	122	134
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 524	$ 932
Net Par Outstanding, Credit Derivative	4	5
Total	$ 528	$ 937
Number of Risks, Financial Guaranty Insurance | risk	27	39
Number of Risks, Credit Derivative | risk	1	1
Number of Risks | risk	28	40
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 5,486	$ 6,090
Net Par Outstanding, Credit Derivative	68	75
Total	$ 5,554	$ 6,165
Number of Risks, Financial Guaranty Insurance | risk	135	145
Number of Risks, Credit Derivative | risk	8	8
Number of Risks | risk	143	153