Reinsurance and Other Monoline Exposures - Exposure by Reinsurer (Details) $ in Millions	Mar. 31, 2016 USD ($)
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	$ 14,153	[1]
Second-to- Pay Insured Par Outstanding	14,321	[1]
Assumed Par Outstanding	20,183	[1]
American Overseas Reinsurance Company Limited [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	4,960	[1],[2]
Second-to- Pay Insured Par Outstanding	0	[1],[2]
Assumed Par Outstanding	30	[1],[2]
Tokio Marine & Nichido Fire Insurance Co., Ltd. (“Tokio”) [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	4,171	[1],[2]
Second-to- Pay Insured Par Outstanding	0	[1],[2]
Assumed Par Outstanding	0	[1],[2]
Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	2,411	[1],[2]
Second-to- Pay Insured Par Outstanding	1,319	[1],[2]
Assumed Par Outstanding	700	[1],[2]
Mitsui Sumitomo Insurance Co. Ltd. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	1,718	[1],[2]
Second-to- Pay Insured Par Outstanding	0	[1],[2]
Assumed Par Outstanding	0	[1],[2]
Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	700	[1]
Second-to- Pay Insured Par Outstanding	38	[1]
Assumed Par Outstanding	0	[1]
Ambac Assurance Corporation [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	117	[1]
Second-to- Pay Insured Par Outstanding	3,892	[1]
Assumed Par Outstanding	9,589	[1]
National [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1],[3]
Second-to- Pay Insured Par Outstanding	5,139	[1],[3]
Assumed Par Outstanding	5,065	[1],[3]
MBIA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1]
Second-to- Pay Insured Par Outstanding	1,591	[1]
Assumed Par Outstanding	428	[1]
FGIC [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1]
Second-to- Pay Insured Par Outstanding	1,437	[1]
Assumed Par Outstanding	636	[1]
Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1]
Second-to- Pay Insured Par Outstanding	86	[1]
Assumed Par Outstanding	823	[1]
CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1]
Second-to- Pay Insured Par Outstanding	43	[1]
Assumed Par Outstanding	2,784	[1]
Other [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	76	[1],[2]
Second-to- Pay Insured Par Outstanding	776	[1],[2]
Assumed Par Outstanding	128	[1],[2]
Non-affiliated Reinsurers [Member]
Ceded Credit Risk [Line Items]
Funds held under reinsurance agreements	$ 436

[1] Includes par related to insured credit derivatives.

[2] The total collateral posted by all non-affiliated reinsurers required or agreeing to post collateral as of March 31, 2016 was approximately $436 million.

[3] Rated AA+ by KBRA.