Reinsurance and Other Monoline Exposures - Second-to-Pay Insured Par Outstanding by Internal Rating (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	$ 18,469	[1],[2]
Radian [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	21	[1],[2]
Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	1,514	[1],[2]
Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	2	[1],[2]
Ambac [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	4,930	[1],[2]
National Public Finance Guarantee Corporation [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	6,210	[1],[2]
MBIA Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	2,613	[1],[2]
FGIC [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	2,074	[1],[2]
Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	109	[1],[2]
CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	102	[1],[2]
Other [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	894	[1],[2]
Public Finance [Member] | Internal Credit Rating, AAA [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	190	[1]
Public Finance [Member] | Internal Credit Rating, AAA [Member] | Radian [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, AAA [Member] | Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, AAA [Member] | Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, AAA [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	30	[1]
Public Finance [Member] | Internal Credit Rating, AAA [Member] | National Public Finance Guarantee Corporation [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	160	[1]
Public Finance [Member] | Internal Credit Rating, AAA [Member] | MBIA Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, AAA [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, AAA [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, AAA [Member] | CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, AAA [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, AA [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	4,580	[1]
Public Finance [Member] | Internal Credit Rating, AA [Member] | Radian [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, AA [Member] | Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	45	[1]
Public Finance [Member] | Internal Credit Rating, AA [Member] | Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	1	[1]
Public Finance [Member] | Internal Credit Rating, AA [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	1,301	[1]
Public Finance [Member] | Internal Credit Rating, AA [Member] | National Public Finance Guarantee Corporation [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	2,193	[1]
Public Finance [Member] | Internal Credit Rating, AA [Member] | MBIA Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	65	[1]
Public Finance [Member] | Internal Credit Rating, AA [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	77	[1]
Public Finance [Member] | Internal Credit Rating, AA [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, AA [Member] | CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	4	[1]
Public Finance [Member] | Internal Credit Rating, AA [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	894	[1]
Public Finance [Member] | Internal Credit Rating, A [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	8,039	[1]
Public Finance [Member] | Internal Credit Rating, A [Member] | Radian [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	3	[1]
Public Finance [Member] | Internal Credit Rating, A [Member] | Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	326	[1]
Public Finance [Member] | Internal Credit Rating, A [Member] | Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, A [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	2,597	[1]
Public Finance [Member] | Internal Credit Rating, A [Member] | National Public Finance Guarantee Corporation [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	3,833	[1]
Public Finance [Member] | Internal Credit Rating, A [Member] | MBIA Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	254	[1]
Public Finance [Member] | Internal Credit Rating, A [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	975	[1]
Public Finance [Member] | Internal Credit Rating, A [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, A [Member] | CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	51	[1]
Public Finance [Member] | Internal Credit Rating, A [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, BBB [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	2,104	[1]
Public Finance [Member] | Internal Credit Rating, BBB [Member] | Radian [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	12	[1]
Public Finance [Member] | Internal Credit Rating, BBB [Member] | Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	727	[1]
Public Finance [Member] | Internal Credit Rating, BBB [Member] | Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	1	[1]
Public Finance [Member] | Internal Credit Rating, BBB [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	637	[1]
Public Finance [Member] | Internal Credit Rating, BBB [Member] | National Public Finance Guarantee Corporation [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, BBB [Member] | MBIA Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	424	[1]
Public Finance [Member] | Internal Credit Rating, BBB [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	281	[1]
Public Finance [Member] | Internal Credit Rating, BBB [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | Internal Credit Rating, BBB [Member] | CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	22	[1]
Public Finance [Member] | Internal Credit Rating, BBB [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | BIG [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	672	[1]
Public Finance [Member] | BIG [Member] | Radian [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	6	[1]
Public Finance [Member] | BIG [Member] | Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	276	[1]
Public Finance [Member] | BIG [Member] | Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | BIG [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	63	[1]
Public Finance [Member] | BIG [Member] | National Public Finance Guarantee Corporation [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | BIG [Member] | MBIA Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | BIG [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	302	[1]
Public Finance [Member] | BIG [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Public Finance [Member] | BIG [Member] | CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	25	[1]
Public Finance [Member] | BIG [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AAA [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	467	[1]
Structured Finance [Member] | Internal Credit Rating, AAA [Member] | Radian [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AAA [Member] | Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	96	[1]
Structured Finance [Member] | Internal Credit Rating, AAA [Member] | Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AAA [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AAA [Member] | National Public Finance Guarantee Corporation [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AAA [Member] | MBIA Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AAA [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	371	[1]
Structured Finance [Member] | Internal Credit Rating, AAA [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AAA [Member] | CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AAA [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AA [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	1,542	[1]
Structured Finance [Member] | Internal Credit Rating, AA [Member] | Radian [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AA [Member] | Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AA [Member] | Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AA [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	1	[1]
Structured Finance [Member] | Internal Credit Rating, AA [Member] | National Public Finance Guarantee Corporation [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AA [Member] | MBIA Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	1,508	[1]
Structured Finance [Member] | Internal Credit Rating, AA [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AA [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	33	[1]
Structured Finance [Member] | Internal Credit Rating, AA [Member] | CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, AA [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, A [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	113	[1]
Structured Finance [Member] | Internal Credit Rating, A [Member] | Radian [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, A [Member] | Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, A [Member] | Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, A [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	64	[1]
Structured Finance [Member] | Internal Credit Rating, A [Member] | National Public Finance Guarantee Corporation [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	24	[1]
Structured Finance [Member] | Internal Credit Rating, A [Member] | MBIA Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, A [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	25	[1]
Structured Finance [Member] | Internal Credit Rating, A [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, A [Member] | CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, A [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, BBB [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	474	[1]
Structured Finance [Member] | Internal Credit Rating, BBB [Member] | Radian [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, BBB [Member] | Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, BBB [Member] | Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, BBB [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	231	[1]
Structured Finance [Member] | Internal Credit Rating, BBB [Member] | National Public Finance Guarantee Corporation [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, BBB [Member] | MBIA Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	243	[1]
Structured Finance [Member] | Internal Credit Rating, BBB [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, BBB [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, BBB [Member] | CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | Internal Credit Rating, BBB [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | BIG [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	288	[1]
Structured Finance [Member] | BIG [Member] | Radian [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | BIG [Member] | Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	44	[1]
Structured Finance [Member] | BIG [Member] | Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | BIG [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	6	[1]
Structured Finance [Member] | BIG [Member] | National Public Finance Guarantee Corporation [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | BIG [Member] | MBIA Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	119	[1]
Structured Finance [Member] | BIG [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	43	[1]
Structured Finance [Member] | BIG [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	76	[1]
Structured Finance [Member] | BIG [Member] | CIFG Assurance North America Inc. [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	0	[1]
Structured Finance [Member] | BIG [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Second-to- Pay Insured Par Outstanding	$ 0	[1]

[1] Assured Guaranty’s internal rating.

[2] Includes par related to insured credit derivatives.