Outstanding Exposure - Narrative (Details) - USD ($)				1 Months Ended	3 Months Ended
	Apr. 28, 2017	Jan. 01, 2017	Jul. 01, 2016	Mar. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Mar. 04, 2016	Dec. 24, 2015	Sep. 15, 2015
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding				$ 446,186,000,000	$ 446,186,000,000	$ 437,535,000,000
Net par amount outstanding				298,243,000,000	298,243,000,000	296,318,000,000
Net par outstanding, European exposure				2,252,000,000	2,252,000,000
Excess Of Loss Reinsurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding				444,000,000	444,000,000	390,000,000
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding				23,690,000,000	23,690,000,000	28,088,000,000
Net par amount outstanding				20,850,000,000	20,850,000,000	25,139,000,000
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding				422,496,000,000	422,496,000,000	409,447,000,000
Net par amount outstanding				277,393,000,000	277,393,000,000	271,179,000,000
Aircraft Residual Value Insurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding				14,000,000	14,000,000
Commitments to provide reinsurance				28,000,000	$ 28,000,000
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Maximum period of liquidity claims (in years)					1 year
Net par amount outstanding				13,228,000,000	$ 13,228,000,000	13,074,000,000
Net par outstanding, European exposure				668,000,000	668,000,000
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding				3,956,000,000	3,956,000,000	4,352,000,000
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding				9,272,000,000	9,272,000,000	8,722,000,000
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding				11,769,000,000	11,769,000,000	15,491,000,000
Ireland [Member] | Mortgage Loans on Real Estate [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding				36,000,000	36,000,000	36,000,000
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding				5,357,000,000	5,357,000,000	5,637,000,000
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding				18,446,000,000	18,446,000,000	22,057,000,000
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding				238,050,000,000	238,050,000,000	244,798,000,000
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding				3,009,000,000	3,009,000,000	3,151,000,000
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding				3,707,000,000	3,707,000,000	4,059,000,000
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding				7,199,000,000	7,199,000,000	7,380,000,000
United States [Member] | AAA [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding				6,765,000,000	6,765,000,000	9,757,000,000
United States [Member] | AAA [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding				1,816,000,000	1,816,000,000	2,066,000,000
Select European Countries [Member]
Schedule of Insured Financial Obligations [Line Items]
Indirect exposure to credits				78,000,000	78,000,000
Indirect European exposure				1,600,000,000	1,600,000,000
Greece [Member] | Pooled corporate obligations [Member]
Schedule of Insured Financial Obligations [Line Items]
Indirect exposure to credits				2,000,000	2,000,000
Indirect European exposure				73,000,000	73,000,000
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding				8,212,000,000	8,212,000,000
Net par amount outstanding				4,937,000,000	4,937,000,000	$ 4,786,000,000
Insured Financial Obligations, 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	4,900,000,000
Turkey [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure				202,000,000	202,000,000
Turkey [Member] | Diversified Payment Rights [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure				202,000,000	202,000,000
Turkey [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure				0	0
Commitment to Provide Guarantees [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties				8,000,000	8,000,000
Commitment to Provide Guarantees [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties				561,000,000	561,000,000
Commitments due before expiration date				$ 438,000,000	$ 438,000,000
Minimum [Member]
Schedule of Insured Financial Obligations [Line Items]
Probability of paying more claims than being reimbursed (as a percent)				50.00%	50.00%
Minimum [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Constant discount rate (as a percent)				4.00%	4.00%
Maximum [Member] | Excess Of Loss Reinsurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding				$ 1,300,000,000	$ 1,300,000,000
Maximum [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Constant discount rate (as a percent)				5.00%	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	$ 120,000,000
Restructuring Support Agreement [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
RSA relending financing in 2017					18,000,000
RSA principal payment deferrals 2018 through 2023					120,000,000
RSA Relending bonds in 2016					41,000,000
AGM [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Bonds issued in settlement			$ 26,000,000
PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Payments made to public on outstanding principal amounts		$ 18,000,000	$ 41,000,000
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
Settlement agreement, required spending, payable to contractor							$ 140,000,000
PRASA (Puerto Rico Aqueduct and Sewer Authority) [Member] | Restructuring Support Agreement [Member] | Puerto Rico [Member] | Subsequent Event [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	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	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	$ 99,000,000
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
Bridge Loan [Member] | AGC [Member] | Restructuring Support Agreement [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Guarantor obligations, debt financing amount								2,000,000
Bridge Loan [Member] | AGM [Member] | Restructuring Support Agreement [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Guarantor obligations, debt financing amount								$ 13,000,000