Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Jun. 30, 2017 USD ($) risk	Dec. 31, 2016 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Credit Derivative	$ 10,106	$ 16,997
Net par amount outstanding	290,620	296,318
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	12,262	12,101
Net Par Outstanding, Credit Derivative	739	973
Net par amount outstanding	$ 13,001	$ 13,074
Number of Risks, Financial Guaranty Insurance | risk	370	392
Number of Risks, Credit Derivative | risk	22	25
Number of Risks | risk	392	417
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 4,377	$ 3,861
Net Par Outstanding, Credit Derivative	548	634
Net par amount outstanding	$ 4,925	$ 4,495
Number of Risks, Financial Guaranty Insurance | risk	157	165
Number of Risks, Credit Derivative | risk	11	10
Number of Risks | risk	168	175
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 1,412	$ 3,857
Net Par Outstanding, Credit Derivative	61	202
Net par amount outstanding	$ 1,473	$ 4,059
Number of Risks, Financial Guaranty Insurance | risk	60	79
Number of Risks, Credit Derivative | risk	3	6
Number of Risks | risk	63	85
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 6,473	$ 4,383
Net Par Outstanding, Credit Derivative	130	137
Net par amount outstanding	$ 6,603	$ 4,520
Number of Risks, Financial Guaranty Insurance | risk	153	148
Number of Risks, Credit Derivative | risk	8	9
Number of Risks | risk	161	157