Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Mar. 31, 2018 USD ($) risk	Dec. 31, 2017 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Credit Derivative	$ 5,615	$ 6,207
Total	257,089	264,952
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	11,341	11,900
Net Par Outstanding, Credit Derivative	258	338
Total	$ 11,599	$ 12,238
Number of Risks, Financial Guaranty Insurance | risk	326	335
Number of Risks, Credit Derivative | risk	16	19
Number of Risks | risk	342	354
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 3,822	$ 4,301
Net Par Outstanding, Credit Derivative	158	227
Total	$ 3,980	$ 4,528
Number of Risks, Financial Guaranty Insurance | risk	134	139
Number of Risks, Credit Derivative | risk	5	7
Number of Risks | risk	139	146
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 1,024	$ 1,344
Net Par Outstanding, Credit Derivative	17	17
Total	$ 1,041	$ 1,361
Number of Risks, Financial Guaranty Insurance | risk	43	46
Number of Risks, Credit Derivative | risk	3	3
Number of Risks | risk	46	49
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 6,495	$ 6,255
Net Par Outstanding, Credit Derivative	83	94
Total	$ 6,578	$ 6,349
Number of Risks, Financial Guaranty Insurance | risk	149	150
Number of Risks, Credit Derivative | risk	8	9
Number of Risks | risk	157	159