Outstanding Exposure - Narrative (Details) € in Millions				9 Months Ended
		Jul. 31, 2015 USD ($)	Jul. 01, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2015 EUR (€)	Sep. 30, 2015 USD ($)		Sep. 09, 2015 USD ($)	Aug. 03, 2015 USD ($)	Dec. 31, 2014 USD ($)		Aug. 14, 2014
Schedule of Insured Financial Obligations [Line Items]
Probability of paying more claims than being reimbursed (as a percent)					50.00%	50.00%
Net Debt Service Outstanding						$ 556,381,000,000				$ 609,622,000,000
Indirect European exposure						4,800,000,000
Net par amount outstanding						372,361,000,000	[1],[2]			403,729,000,000	[3]
Surety reinsurance contracts [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding | €					€ 12
Pooled corporate obligations [Member]
Schedule of Insured Financial Obligations [Line Items]
Indirect European exposure						333,000,000
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding						46,736,000,000				56,010,000,000
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding						509,645,000,000				553,612,000,000
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Maximum period of liquidity claims (in years)				1 year
Net par amount outstanding						17,611,000,000				18,247,000,000
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						23,527,000,000				30,143,000,000
United Kingdom and Ireland [Member] | Mortgage Loans on Real Estate [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding						105,000,000				127,000,000
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						8,400,000,000				9,417,000,000
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						35,435,000,000	[1],[2]			41,171,000,000	[3]
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						300,732,000,000	[1],[2]			322,123,000,000	[3]
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						4,130,000,000				5,643,000,000
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						6,058,000,000				8,186,000,000
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						9,480,000,000				7,850,000,000
United States [Member] | AAA [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						16,730,000,000	[4]			20,037,000,000
Net par outstanding updated from BIG						1,351,000,000
United States [Member] | AAA [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						3,067,000,000				4,082,000,000
Select European Countries [Member]
Schedule of Insured Financial Obligations [Line Items]
Indirect European exposure						278,000,000
Greece [Member] | Pooled corporate obligations [Member]
Schedule of Insured Financial Obligations [Line Items]
Indirect European exposure						8,000,000
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding						8,641,000,000
Net par amount outstanding						5,091,000,000	[5]			4,939,000,000
Puerto Rico [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						5,100,000,000
Commitment to Provide Guarantees [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties						504,000,000
Commitments due before expiration date						357,000,000
Other commitments, future minimum payments, Remainder of Fiscal Year						$ 23,000,000
Minimum [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Expiration date for insured financial obligation commitments				Oct. 01, 2015
Minimum [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Constant discount rate (as a percent)					4.50%	4.50%
Maximum [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Expiration date for insured financial obligation commitments				Feb. 25, 2017
Maximum [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Constant discount rate (as a percent)					5.00%	5.00%
Radian [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	[1],[2]					$ 12,400,000,000
Radian [Member] | Puerto Rico [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Increase in net par due to acquisition				$ 385,000,000
PREPA [Member] | Radian [Member] | Puerto Rico [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Increase in net par due to acquisition				21,000,000
Puerto Rico Highways and Transportation Authority [Member] [Member] | Radian [Member] | Puerto Rico [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Increase in net par due to acquisition				$ 166,000,000
Subject to the Terms of the Recovery Act [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding						4,541,000,000
Net par amount outstanding						2,575,000,000	[5]			2,447,000,000
Subject to the Terms of the Recovery Act [Member] | PRHTA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						909,000,000	[5]			844,000,000
Subject to the Terms of the Recovery Act [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						744,000,000	[5]			772,000,000
Payments received on par scheduled to be paid			$ 416,000,000
Not Subject to the Terms of the Recovery Act [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding						4,100,000,000
Net par amount outstanding						2,516,000,000	[5]			2,492,000,000
Not Subject to the Terms of the Recovery Act [Member] | Puerto Rico Municipal Finance Authority [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						387,000,000	[5]			$ 399,000,000
Working Group For The Fiscal And Economic Recovery Of Puerto Rico [Member] | Subject to the Terms of the Recovery Act [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Cumulative Financing Gap, Amount Before Corrective Action								$ 27,800,000,000
Cumulative Financing Gap, Net Of Corrective Action								$ 14,000,000,000
PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Percentage of outstanding bonds of PREPA												60.00%
PREPA [Member] | Subject to the Terms of the Recovery Act [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Bonds purchased in settlement			$ 131,000,000
Puerto Rico Public Finance Corporation [Member] | Subject to the Terms of the Recovery Act [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Overdue debt service payment									$ 58,000,000
AGM and AGC [Member] | Subject to the Terms of the Recovery Act [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Bonds bought in settlement		$ 74,000,000
PRHTA [Member] | Subject to the Terms of the Recovery Act [Member] | Minimum [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Special assessment bond, contingently issuable						1,000,000,000
PRHTA [Member] | Subject to the Terms of the Recovery Act [Member] | Maximum [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Special assessment bond, contingently issuable						$ 2,950,000,000.00

[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] (3)Includes $1,351 million net par in the form of CDS that was upgraded from BIG as of September 30, 2015, in anticipation of the termination of such CDS that occurred early in the fourth quarter of 2015. In the fourth quarter of 2015, the exposure will be removed.

[5] $385 million acquired in the Radian Asset Acquisition, of which $21 million was of PREPA and $166 million