Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Sep. 30, 2017 USD ($) risk	Dec. 31, 2016 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Credit Derivative	$ 7,535	$ 16,997
Total	275,767	296,318
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	12,530	12,101
Net Par Outstanding, Credit Derivative	696	973
Total	$ 13,226	$ 13,074
Number of Risks, Financial Guaranty Insurance | risk	349	392
Number of Risks, Credit Derivative | risk	21	25
Number of Risks | risk	370	417
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 4,628	$ 3,861
Net Par Outstanding, Credit Derivative	544	634
Total	$ 5,172	$ 4,495
Number of Risks, Financial Guaranty Insurance | risk	150	165
Number of Risks, Credit Derivative | risk	9	10
Number of Risks | risk	159	175
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 1,382	$ 3,857
Net Par Outstanding, Credit Derivative	65	202
Total	$ 1,447	$ 4,059
Number of Risks, Financial Guaranty Insurance | risk	48	79
Number of Risks, Credit Derivative | risk	4	6
Number of Risks | risk	52	85
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 6,520	$ 4,383
Net Par Outstanding, Credit Derivative	87	137
Total	$ 6,607	$ 4,520
Number of Risks, Financial Guaranty Insurance | risk	151	148
Number of Risks, Credit Derivative | risk	8	9
Number of Risks | risk	159	157