Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Mar. 31, 2019 USD ($) risk	Dec. 31, 2018 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Credit Derivative	$ 6,120	$ 6,182
Total	237,325	241,802
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	9,473	10,003
Net Par Outstanding, Credit Derivative	153	157
Total	$ 9,626	$ 10,160
Number of Risks, Financial Guaranty Insurance | risk	301	312
Number of Risks, Credit Derivative | risk	15	15
Number of Risks | risk	316	327
BIG [Member] | BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 3,191	$ 2,981
Net Par Outstanding, Credit Derivative	77	77
Total	$ 3,268	$ 3,058
Number of Risks, Financial Guaranty Insurance | risk	128	128
Number of Risks, Credit Derivative | risk	6	6
Number of Risks | risk	134	134
BIG [Member] | BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 573	$ 932
Net Par Outstanding, Credit Derivative	5	5
Total	$ 578	$ 937
Number of Risks, Financial Guaranty Insurance | risk	31	39
Number of Risks, Credit Derivative | risk	1	1
Number of Risks | risk	32	40
BIG [Member] | BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 5,709	$ 6,090
Net Par Outstanding, Credit Derivative	71	75
Total	$ 5,780	$ 6,165
Number of Risks, Financial Guaranty Insurance | risk	142	145
Number of Risks, Credit Derivative | risk	8	8
Number of Risks | risk	150	153