Outstanding Exposure - Puerto Rico Net Par Outstanding (Details) - USD ($) $ in Millions	Sep. 30, 2015		Dec. 31, 2014
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 372,361	[1],[2]	$ 403,729	[3]
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	5,091	[4]	4,939
Puerto Rico [Member] | Subject to the Terms of the Recovery Act [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,575	[4]	2,447
Puerto Rico [Member] | Subject to the Terms of the Recovery Act [Member] | PRHTA (Transportation revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	909	[4]	844
Puerto Rico [Member] | Subject to the Terms of the Recovery Act [Member] | PREPA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	744	[4]	772
Puerto Rico [Member] | Subject to the Terms of the Recovery Act [Member] | Puerto Rico Aqueduct and Sewer Authority [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	388	[4]	384
Puerto Rico [Member] | Subject to the Terms of the Recovery Act [Member] | PRHTA (Highway revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	370	[4]	273
Puerto Rico [Member] | Subject to the Terms of the Recovery Act [Member] | Puerto Rico Convention Center District Authority [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	164	[4]	174
Puerto Rico [Member] | Not Subject to the Terms of the Recovery Act [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	2,516	[4]	2,492
Puerto Rico [Member] | Not Subject to the Terms of the Recovery Act [Member] | Commonwealth of Puerto Rico - General Obligation Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	1,620	[4]	1,672
Puerto Rico [Member] | Not Subject to the Terms of the Recovery Act [Member] | Puerto Rico Municipal Finance Agency [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	387	[4]	399
Puerto Rico [Member] | Not Subject to the Terms of the Recovery Act [Member] | Puerto Rico Sales Tax Financing Corporation [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	269	[4]	269
Puerto Rico [Member] | Not Subject to the Terms of the Recovery Act [Member] | Puerto Rico Public Buildings Authority [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	188	[4]	100
Puerto Rico [Member] | Not Subject to the Terms of the Recovery Act [Member] | GDB [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	33	[4]	33
Puerto Rico [Member] | Not Subject to the Terms of the Recovery Act [Member] | PRIFA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	18	[4]	18
Puerto Rico [Member] | Not Subject to the Terms of the Recovery Act [Member] | University of Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	$ 1	[4]	$ 1

[1] Excludes $1.6 billion of loss mitigation securities insured and held by the Company as of September 30, 2015, which are primarily BIG.

[2] The September 30, 2015 amounts include $12.4 billion of net par acquired from Radian Asset.

[3] Excludes $1.3 billion of loss mitigation securities insured and held by the Company as of December 31, 2014, which are primarily BIG.

[4] $385 million acquired in the Radian Asset Acquisition, of which $21 million was of PREPA and $166 million