Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Jun. 30, 2018 USD ($) risk	Dec. 31, 2017 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Credit Derivative	$ 6,681	$ 6,207
Total	257,804	264,952
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	10,385	11,900
Net Par Outstanding, Credit Derivative	253	338
Total	$ 10,638	$ 12,238
Number of Risks, Financial Guaranty Insurance | risk	332	335
Number of Risks, Credit Derivative | risk	18	19
Number of Risks | risk	350	354
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 2,734	$ 4,301
Net Par Outstanding, Credit Derivative	156	227
Total	$ 2,890	$ 4,528
Number of Risks, Financial Guaranty Insurance | risk	138	139
Number of Risks, Credit Derivative | risk	5	7
Number of Risks | risk	143	146
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 1,096	$ 1,344
Net Par Outstanding, Credit Derivative	16	17
Total	$ 1,112	$ 1,361
Number of Risks, Financial Guaranty Insurance | risk	44	46
Number of Risks, Credit Derivative | risk	3	3
Number of Risks | risk	47	49
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 6,555	$ 6,255
Net Par Outstanding, Credit Derivative	81	94
Total	$ 6,636	$ 6,349
Number of Risks, Financial Guaranty Insurance | risk	150	150
Number of Risks, Credit Derivative | risk	10	9
Number of Risks | risk	160	159