Outstanding Exposure - Narrative (Details)		1 Months Ended	6 Months Ended
	Apr. 28, 2017	Mar. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) judget	Dec. 31, 2016 USD ($)	Sep. 15, 2015 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			$ 436,045,000,000	$ 437,535,000,000
Net par amount outstanding			$ 290,620,000,000	296,318,000,000
Number of Federal Judges | judget			5
Net par outstanding, European exposure			$ 2,365,000,000
Excess Of Loss Reinsurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			506,000,000	390,000,000
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			20,356,000,000	28,088,000,000
Net par amount outstanding			17,669,000,000	25,139,000,000
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			415,689,000,000	409,447,000,000
Net par amount outstanding			272,951,000,000	271,179,000,000
Aircraft Residual Value Insurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			127,000,000
Commitments to provide reinsurance			$ 46,000,000
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Maximum period of liquidity claims (in years)			1 year
Net par amount outstanding			$ 13,001,000,000	13,074,000,000
Net par outstanding, European exposure			709,000,000
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			3,714,000,000	4,352,000,000
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			9,287,000,000	8,722,000,000
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			9,053,000,000	15,491,000,000
Ireland [Member] | Mortgage Loans on Real Estate [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			39,000,000	36,000,000
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			5,089,000,000	5,637,000,000
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			15,655,000,000	22,057,000,000
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			232,418,000,000	244,798,000,000
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			2,875,000,000	3,151,000,000
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			3,553,000,000	4,059,000,000
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			7,142,000,000	7,380,000,000
United States [Member] | AAA [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			4,364,000,000	9,757,000,000
United States [Member] | AAA [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			1,709,000,000	2,066,000,000
Select European Countries [Member]
Schedule of Insured Financial Obligations [Line Items]
Indirect exposure to credits			50,000,000
Indirect European exposure			1,100,000,000
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			8,210,000,000
Net par amount outstanding			4,937,000,000	$ 4,786,000,000
Certified fiscal plan, commonwealth debt service		$ 7,900,000,000
Puerto Rico [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			4,900,000,000
Turkey [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure			201,000,000
Turkey [Member] | Diversified Payment Rights [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure			201,000,000
Turkey [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure			0
Commitment to Provide Guarantees [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties			14,000,000
Commitment to Provide Guarantees [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties			461,000,000
Commitments due before expiration date			$ 191,000,000
Minimum [Member]
Schedule of Insured Financial Obligations [Line Items]
Probability of paying more claims than being reimbursed (as a percent)			50.00%
Minimum [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Constant discount rate (as a percent)			4.00%
Maximum [Member] | Excess Of Loss Reinsurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			$ 1,300,000,000
Maximum [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Constant discount rate (as a percent)			5.00%
PRHTA (Highway revenue) [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Obligations secured by taxes on crude oil, unfinished oil and derivative products			$ 120,000,000
Restructuring Support Agreement [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
RSA principal payment deferrals 2018 through 2023			120,000,000
PRASA (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
PRASA (Puerto Rico Aqueduct and Sewer Authority) [Member] | Restructuring Support Agreement [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Expected repayments of debt, percent	65.00%
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
Surety Bond [Member] | AGC [Member] | Restructuring Support Agreement [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Possible liquidity claims, gross exposure			14,000,000
Surety Bond [Member] | AGM [Member] | Restructuring Support Agreement [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Possible liquidity claims, gross exposure			$ 99,000,000