Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Sep. 30, 2019 USD ($) risk	Dec. 31, 2018 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Credit Derivative	$ 6,165	$ 6,182
Total	229,375	241,802
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	8,383	10,003
Net Par Outstanding, Credit Derivative	132	157
Total	$ 8,515	$ 10,160
Number of Risks, Financial Guaranty Insurance | risk	279	312
Number of Risks, Credit Derivative | risk	14	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	$ 2,543	$ 2,981
Net Par Outstanding, Credit Derivative	69	77
Total	$ 2,612	$ 3,058
Number of Risks, Financial Guaranty Insurance | risk	119	128
Number of Risks, Credit Derivative | risk	6	6
Number of Risks | risk	125	134
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 547	$ 932
Net Par Outstanding, Credit Derivative	4	5
Total	$ 551	$ 937
Number of Risks, Financial Guaranty Insurance | risk	26	39
Number of Risks, Credit Derivative | risk	1	1
Number of Risks | risk	27	40
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 5,293	$ 6,090
Net Par Outstanding, Credit Derivative	59	75
Total	$ 5,352	$ 6,165
Number of Risks, Financial Guaranty Insurance | risk	134	145
Number of Risks, Credit Derivative | risk	7	8
Number of Risks | risk	141	153