Outstanding Exposure - Narrative (Details) - USD ($)						3 Months Ended
		Apr. 30, 2016	Jan. 01, 2016	Jul. 31, 2015	Jul. 01, 2015	Mar. 31, 2016		Mar. 04, 2016	Dec. 31, 2015		Dec. 24, 2015	Sep. 15, 2015
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding						$ 516,399,000,000			$ 536,341,000,000
Indirect European exposure						4,100,000,000
Net par amount outstanding						347,015,000,000	[1]		358,571,000,000	[2]
Pooled corporate obligations [Member]
Schedule of Insured Financial Obligations [Line Items]
Indirect European exposure						231,000,000
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding						40,037,000,000			41,915,000,000
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding						$ 476,362,000,000			494,426,000,000
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Maximum period of liquidity claims (in years)						1 year
Net par amount outstanding						$ 15,117,000,000			15,183,000,000
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						19,711,000,000			20,837,000,000
Ireland [Member] | Mortgage Loans on Real Estate [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding						107,000,000			102,000,000
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						6,677,000,000			7,067,000,000
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						30,452,000,000	[1]		31,770,000,000	[2]
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						282,055,000,000	[1]		291,866,000,000	[2]
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						3,682,000,000			3,973,000,000
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						5,131,000,000			5,469,000,000
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						7,953,000,000			7,784,000,000
United States [Member] | AAA [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						13,953,000,000			14,366,000,000
United States [Member] | AAA [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						2,541,000,000			3,053,000,000
Select European Countries [Member]
Schedule of Insured Financial Obligations [Line Items]
Indirect European exposure						206,000,000
Greece [Member] | Pooled corporate obligations [Member]
Schedule of Insured Financial Obligations [Line Items]
Indirect European exposure						6,000,000
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding						8,469,000,000
Net par amount outstanding						5,054,000,000			5,053,000,000
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						240,000,000
Commitments due before expiration date						66,000,000
Other commitments, future minimum payments, Remainder of Fiscal Year						$ 110,000,000
Minimum [Member]
Schedule of Insured Financial Obligations [Line Items]
Probability of paying more claims than being reimbursed (as a percent)						50.00%
Minimum [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Expiration date for insured financial obligation commitments						Apr. 01, 2016
Minimum [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Constant discount rate (as a percent)						4.00%
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%
PRIFA [Member] | Puerto Rico [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Payment On Outstanding Principal Amount			$ 451,000
Subject to the Terms of the Recovery Act [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding						$ 4,476,000,000
Net par amount outstanding						2,575,000,000			2,575,000,000
Subject to the Terms of the Recovery Act [Member] | PRHTA (Highway revenue) [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	[3]					369,000,000			370,000,000
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	[3]					910,000,000			909,000,000
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			744,000,000
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						3,993,000,000
Net par amount outstanding						2,479,000,000			2,478,000,000
Not Subject to the Terms of the Recovery Act [Member] | PRIFA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding	[3],[4]					18,000,000			18,000,000
Not Subject to the Terms of the Recovery Act [Member] | MFA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						387,000,000			387,000,000
Not Subject to the Terms of the Recovery Act [Member] | Puerto Rico Sales Tax Financing Corporation [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding						$ 270,000,000			$ 269,000,000
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
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
Puerto Rico Aqueduct and Sewer Authority [Member] | U.S. Justice Department and U.S. Environmental Protection Agency [Member]
Schedule of Insured Financial Obligations [Line Items]
Settlement Agreement, Required Spending Threshold												$ 1,600,000,000
Settlement Agreement, Required Spending, Payable To Contractor								$ 140,000,000
Surety Bond [Member] | Restructuring Support Agreement [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Possible liquidity claims, gross exposure											$ 113,000,000
Bridge Loan [Member] | Restructuring Support Agreement [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Guarantor Obligations, Debt Financing Amount											$ 15,000,000
Subsequent Event [Member] | Government Development Bank [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Insured Financial Obligations, Insured Contractual Payments Under Moratorium, Amount		$ 422,000,000
Insured Financial Obligations, Debt Restructuring, Initial Litigation Period Forgone		30 days

[1] Excludes $1.5 billion of loss mitigation securities insured and held by the Company as of March 31, 2016, which are primarily BIG.

[2] Excludes $1.5 billion of loss mitigation securities insured and held by the Company as of December 31, 2015, which are primarily BIG.

[3] The Governor issued executive orders on November 30, 2015, and December 8, 2015, directing the Puerto Rico Department of Treasury and the Puerto Rico Tourism Company to retain or transfer certain taxes and revenues pledged to secure the payment of bonds issued by PRHTA, PRIFA and PRCCDA. On January 7, 2016 the Company sued various Puerto Rico governmental officials in the United States District Court, District of Puerto Rico asserting that this attempt to “claw back” pledged taxes and revenues is unconstitutional, and demanding declaratory and injunctive relief.

[4] On January 1, 2016 PRIFA defaulted on full payment of a portion of the interest due on its bonds on that date. For those PRIFA bonds the Company had insured, the Company paid approximately $451 thousand of claims for the interest payments on which PRIFA had defaulted.