Reinsurance and Other Monoline Exposures - Exposure by Reinsurer (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	$ 21,987	[1]
Second-to- Pay Insured Par Outstanding	17,811	[1]
Assumed Par Outstanding	26,266	[1]
American Overseas Reinsurance Company Limited (f/k/a Ram Re) [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	6,363	[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	5,006	[1],[2]
Second-to- Pay Insured Par Outstanding	0	[1],[2]
Assumed Par Outstanding	0	[1],[2]
Radian [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	3,984	[1],[3]
Second-to- Pay Insured Par Outstanding	18	[1],[3]
Assumed Par Outstanding	620	[1],[3]
Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	3,522	[1]
Second-to- Pay Insured Par Outstanding	1,451	[1]
Assumed Par Outstanding	160	[1]
Mitsui Sumitomo Insurance Co. Ltd. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	1,995	[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	745	[1]
Second-to- Pay Insured Par Outstanding	1	[1]
Assumed Par Outstanding	0	[1]
Swiss Reinsurance Co. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	91	[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	4,796	[1]
Assumed Par Outstanding	13,281	[1]
National Public Finance Guarantee Corporation [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1],[4]
Second-to- Pay Insured Par Outstanding	6,061	[1],[4]
Assumed Par Outstanding	5,724	[1],[4]
MBIA Inc. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1],[5]
Second-to- Pay Insured Par Outstanding	2,465	[1],[5]
Assumed Par Outstanding	484	[1],[5]
Financial Guaranty Insurance Co [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1]
Second-to- Pay Insured Par Outstanding	2,008	[1]
Assumed Par Outstanding	809	[1]
Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0	[1]
Second-to- Pay Insured Par Outstanding	105	[1]
Assumed Par Outstanding	936	[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]
Assumed Par Outstanding	4,177	[1]
Other [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	199	[1]
Second-to- Pay Insured Par Outstanding	804	[1]
Assumed Par Outstanding	$ 45	[1]

[1] Includes par related to insured credit derivatives.

[2] The Company has structural collateral agreements satisfying the triple-A credit requirement of S&P and/or Moody’s.

[3] (4)On April 1, 2015, AGC consummated the acquisition of Radian Asset and merged Radian Asset with and into AGC, with AGC as the surviving company of the merger.

[4] (6)NPFGC is rated AA+ by KBRA.

[5] (7)MBIA includes subsidiaries MBIA Insurance Corp. rated B by S&P and B2 by Moody's and MBIA U.K. Insurance Ltd. rated B by S&P and Ba2 by Moody’s.