Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Dec. 31, 2017 USD ($) risk	Dec. 31, 2016 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Credit Derivative	$ 6,207	$ 16,997
Net Par Outstanding	$ 264,952	296,318
Number of Risks | risk	10,239
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 11,900	12,101
Net Par Outstanding, Credit Derivative	338	973
Net Par Outstanding	$ 12,238	$ 13,074
Number of Risks, Financial Guaranty Insurance | risk	335	392
Number of Risks, Credit Derivative | risk	19	25
Number of Risks | risk	354	417
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 4,301	$ 3,861
Net Par Outstanding, Credit Derivative	227	634
Net Par Outstanding	$ 4,528	$ 4,495
Number of Risks, Financial Guaranty Insurance | risk	139	165
Number of Risks, Credit Derivative | risk	7	10
Number of Risks | risk	146	175
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 1,344	$ 3,857
Net Par Outstanding, Credit Derivative	17	202
Net Par Outstanding	$ 1,361	$ 4,059
Number of Risks, Financial Guaranty Insurance | risk	46	79
Number of Risks, Credit Derivative | risk	3	6
Number of Risks | risk	49	85
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 6,255	$ 4,383
Net Par Outstanding, Credit Derivative	94	137
Net Par Outstanding	$ 6,349	$ 4,520
Number of Risks, Financial Guaranty Insurance | risk	150	148
Number of Risks, Credit Derivative | risk	9	9
Number of Risks | risk	159	157