Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Mar. 31, 2017 USD ($) risk	Dec. 31, 2016 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Credit Derivative	$ 12,856	$ 16,997
Net par amount outstanding	298,243	296,318
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	12,476	12,101
Net Par Outstanding, Credit Derivative	752	973
Net par amount outstanding	$ 13,228	$ 13,074
Number of Risks, Financial Guaranty Insurance | risk	379	392
Number of Risks, Credit Derivative | risk	22	25
Number of Risks | risk	401	417
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 4,427	$ 3,861
Net Par Outstanding, Credit Derivative	487	634
Net par amount outstanding	$ 4,914	$ 4,495
Number of Risks, Financial Guaranty Insurance | risk	159	165
Number of Risks, Credit Derivative | risk	8	10
Number of Risks | risk	167	175
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 3,698	$ 3,857
Net Par Outstanding, Credit Derivative	133	202
Net par amount outstanding	$ 3,831	$ 4,059
Number of Risks, Financial Guaranty Insurance | risk	71	79
Number of Risks, Credit Derivative | risk	5	6
Number of Risks | risk	76	85
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 4,351	$ 4,383
Net Par Outstanding, Credit Derivative	132	137
Net par amount outstanding	$ 4,483	$ 4,520
Number of Risks, Financial Guaranty Insurance | risk	149	148
Number of Risks, Credit Derivative | risk	9	9
Number of Risks | risk	158	157