Reinsurance and Other Monoline Exposures - Exposure by Reinsurer (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	$ 22,975	[1]	$ 28,788
Second-to- Pay Insured Par Outstanding	18,469	[1],[2]
Assumed Par Outstanding	27,886	[1]
American Overseas Reinsurance Company Limited (f/k/a Ram Re) [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	6,727	[1]
Second-to- Pay Insured Par Outstanding	0	[1]
Assumed Par Outstanding	30	[1]
Tokio Marine & Nichido Fire Insurance Co., Ltd. (“Tokio”) [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	5,276	[1]
Second-to- Pay Insured Par Outstanding	0	[1]
Assumed Par Outstanding	0	[1]
Radian [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	4,104	[1]
Second-to- Pay Insured Par Outstanding	21	[1],[2]
Assumed Par Outstanding	671	[1]
Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	3,715	[1]
Second-to- Pay Insured Par Outstanding	1,514	[1],[2]
Assumed Par Outstanding	161	[1]
Mitsui Sumitomo Insurance Co. Ltd. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	2,033	[1]
Second-to- Pay Insured Par Outstanding	0	[1]
Assumed Par Outstanding	0	[1]
Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	746	[1]
Second-to- Pay Insured Par Outstanding	2	[1],[2]
Assumed Par Outstanding	0	[1]
Swiss Reinsurance Co. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	93	[1]
Second-to- Pay Insured Par Outstanding	0	[1]
Assumed Par Outstanding	0	[1]
Ambac [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	82	[1]
Second-to- Pay Insured Par Outstanding	4,930	[1],[2]
Assumed Par Outstanding	14,342	[1]
National Public Finance Guarantee Corporation [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1]
Second-to- Pay Insured Par Outstanding	6,210	[1],[2]
Assumed Par Outstanding	5,894	[1]
MBIA Inc. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1]
Second-to- Pay Insured Par Outstanding	2,613	[1],[2]
Assumed Par Outstanding	587	[1]
FGIC [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1]
Second-to- Pay Insured Par Outstanding	2,074	[1],[2]
Assumed Par Outstanding	834	[1]
Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1]
Second-to- Pay Insured Par Outstanding	109	[1],[2]
Assumed Par Outstanding	956	[1]
CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1]
Second-to- Pay Insured Par Outstanding	102	[1],[2]
Assumed Par Outstanding	4,365	[1]
Other [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	199	[1]
Second-to- Pay Insured Par Outstanding	894	[1],[2]
Assumed Par Outstanding	$ 46	[1]

[1] Includes par related to insured credit derivatives.

[2] Assured Guaranty’s internal rating.