Outstanding Exposure - Narrative (Details)		1 Months Ended	9 Months Ended
	Apr. 28, 2017	Mar. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) judge	Dec. 31, 2016 USD ($)	Sep. 15, 2015 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			$ 416,724,000,000	$ 437,535,000,000
Net par amount outstanding			$ 275,767,000,000	296,318,000,000
Number of Federal Judges | judge			5
Net par outstanding, European exposure			$ 2,553,000,000
Insured Financial Obligations, Loss Mitigation Securities			2,000,000,000	2,100,000,000
Excess Of Loss Reinsurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			540,000,000	390,000,000
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			17,377,000,000	28,088,000,000
Net par amount outstanding			14,824,000,000	25,139,000,000
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			399,347,000,000	409,447,000,000
Net par amount outstanding			260,943,000,000	271,179,000,000
Aircraft Residual Value Insurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			$ 116,000,000
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Maximum period of liquidity claims (in years)			1 year
Net par amount outstanding			$ 13,226,000,000	13,074,000,000
Net par outstanding, European exposure			793,000,000
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			3,611,000,000	4,352,000,000
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			9,615,000,000	8,722,000,000
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			6,190,000,000	15,491,000,000
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			5,064,000,000	5,637,000,000
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			13,142,000,000	22,057,000,000
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			218,216,000,000	244,798,000,000
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			2,869,000,000	3,151,000,000
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			3,454,000,000	4,059,000,000
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			7,334,000,000	7,380,000,000
United States [Member] | AAA [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			2,333,000,000	9,757,000,000
United States [Member] | AAA [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			915,000,000	2,066,000,000
Select European Countries [Member]
Schedule of Insured Financial Obligations [Line Items]
Indirect exposure to credits			46,000,000
Indirect European exposure			700,000,000
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding			8,197,000,000
Net par amount outstanding			4,966,000,000	$ 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			5,000,000,000
U.S. Virgin Islands [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			498,000,000
U.S. Virgin Islands [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			224,000,000
U.S. Virgin Islands [Member] | Internal Investment Grade [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding			274,000,000
Turkey [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure			200,000,000
Turkey [Member] | Diversified Payment Rights [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure			200,000,000
Turkey [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, European exposure			0
Commitment to Provide Guarantees [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties			$ 43,000,000
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			$ 1,200,000,000
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