Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Sep. 30, 2018 USD ($) risk	Dec. 31, 2017 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Credit Derivative	$ 6,475	$ 6,207
Total	246,940	264,952
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	10,051	11,900
Net Par Outstanding, Credit Derivative	242	338
Total	$ 10,293	$ 12,238
Number of Risks, Financial Guaranty Insurance | risk	317	335
Number of Risks, Credit Derivative | risk	16	19
Number of Risks | risk	333	354
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 2,746	$ 4,301
Net Par Outstanding, Credit Derivative	156	227
Total	$ 2,902	$ 4,528
Number of Risks, Financial Guaranty Insurance | risk	130	139
Number of Risks, Credit Derivative | risk	6	7
Number of Risks | risk	136	146
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 968	$ 1,344
Net Par Outstanding, Credit Derivative	8	17
Total	$ 976	$ 1,361
Number of Risks, Financial Guaranty Insurance | risk	41	46
Number of Risks, Credit Derivative | risk	2	3
Number of Risks | risk	43	49
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 6,337	$ 6,255
Net Par Outstanding, Credit Derivative	78	94
Total	$ 6,415	$ 6,349
Number of Risks, Financial Guaranty Insurance | risk	146	150
Number of Risks, Credit Derivative | risk	8	9
Number of Risks | risk	154	159