Reinsurance and Other Monoline Exposures - Exposure by Credit Rating (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Ceded Credit Risk [Line Items]
Ceded Par Amount	$ 22,975	[1]	$ 28,788
Internal Credit Rating, AAA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	1,798
Internal Credit Rating, AA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	5,207
Internal Credit Rating, A [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	7,168
Internal Credit Rating, BBB [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	6,053
BIG [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	2,749
American Overseas Reinsurance Company Limited (f/k/a Ram Re) [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	6,727	[1]
American Overseas Reinsurance Company Limited (f/k/a Ram Re) [Member] | Internal Credit Rating, AAA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	633
American Overseas Reinsurance Company Limited (f/k/a Ram Re) [Member] | Internal Credit Rating, AA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	2,452
American Overseas Reinsurance Company Limited (f/k/a Ram Re) [Member] | Internal Credit Rating, A [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	1,992
American Overseas Reinsurance Company Limited (f/k/a Ram Re) [Member] | Internal Credit Rating, BBB [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	1,158
American Overseas Reinsurance Company Limited (f/k/a Ram Re) [Member] | BIG [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	492
Tokio Marine & Nichido Fire Insurance Co., Ltd. (“Tokio”) [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	5,276	[1]
Tokio Marine & Nichido Fire Insurance Co., Ltd. (“Tokio”) [Member] | Internal Credit Rating, AAA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	763
Tokio Marine & Nichido Fire Insurance Co., Ltd. (“Tokio”) [Member] | Internal Credit Rating, AA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	968
Tokio Marine & Nichido Fire Insurance Co., Ltd. (“Tokio”) [Member] | Internal Credit Rating, A [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	1,485
Tokio Marine & Nichido Fire Insurance Co., Ltd. (“Tokio”) [Member] | Internal Credit Rating, BBB [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	1,281
Tokio Marine & Nichido Fire Insurance Co., Ltd. (“Tokio”) [Member] | BIG [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	779
Radian [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	4,104	[1]
Radian [Member] | Internal Credit Rating, AAA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	206
Radian [Member] | Internal Credit Rating, AA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	287
Radian [Member] | Internal Credit Rating, A [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	2,037
Radian [Member] | Internal Credit Rating, BBB [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	1,085
Radian [Member] | BIG [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	489
Syncora Guarantee Inc. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	3,715	[1]
Syncora Guarantee Inc. [Member] | Internal Credit Rating, AAA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	0
Syncora Guarantee Inc. [Member] | Internal Credit Rating, AA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	291
Syncora Guarantee Inc. [Member] | Internal Credit Rating, A [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	498
Syncora Guarantee Inc. [Member] | Internal Credit Rating, BBB [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	2,193
Syncora Guarantee Inc. [Member] | BIG [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	733
Mitsui Sumitomo Insurance Co. Ltd. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	2,033	[1]
Mitsui Sumitomo Insurance Co. Ltd. [Member] | Internal Credit Rating, AAA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	134
Mitsui Sumitomo Insurance Co. Ltd. [Member] | Internal Credit Rating, AA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	669
Mitsui Sumitomo Insurance Co. Ltd. [Member] | Internal Credit Rating, A [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	742
Mitsui Sumitomo Insurance Co. Ltd. [Member] | Internal Credit Rating, BBB [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	299
Mitsui Sumitomo Insurance Co. Ltd. [Member] | BIG [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	189
Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	746	[1]
Aca Financial Guaranty Corp [Member] | Internal Credit Rating, AAA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	0
Aca Financial Guaranty Corp [Member] | Internal Credit Rating, AA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	458
Aca Financial Guaranty Corp [Member] | Internal Credit Rating, A [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	277
Aca Financial Guaranty Corp [Member] | Internal Credit Rating, BBB [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	11
Aca Financial Guaranty Corp [Member] | BIG [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	0
Swiss Reinsurance Co. [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	93	[1]
Swiss Reinsurance Co. [Member] | Internal Credit Rating, AAA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	0
Swiss Reinsurance Co. [Member] | Internal Credit Rating, AA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	0
Swiss Reinsurance Co. [Member] | Internal Credit Rating, A [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	0
Swiss Reinsurance Co. [Member] | Internal Credit Rating, BBB [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	26
Swiss Reinsurance Co. [Member] | BIG [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	67
Ambac [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	82	[1]
Ambac [Member] | Internal Credit Rating, AAA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	0
Ambac [Member] | Internal Credit Rating, AA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	0
Ambac [Member] | Internal Credit Rating, A [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	82
Ambac [Member] | Internal Credit Rating, BBB [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	0
Ambac [Member] | BIG [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	0
Other [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	199	[1]
Other [Member] | Internal Credit Rating, AAA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	62
Other [Member] | Internal Credit Rating, AA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	82
Other [Member] | Internal Credit Rating, A [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	55
Other [Member] | Internal Credit Rating, BBB [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	0
Other [Member] | BIG [Member]
Ceded Credit Risk [Line Items]
Ceded Par Amount	$ 0

[1] Includes par related to insured credit derivatives.