Outstanding Exposure - Narrative (Details)		3 Months Ended
	Apr. 20, 2018	Mar. 31, 2018 USD ($) judge	Apr. 19, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 15, 2015 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		$ 257,089,000,000		$ 264,952,000,000
Number of Federal Judges | judge		5
Insured Financial Obligations, Loss Mitigation Securities		$ 2,000,000,000		2,000,000,000
Excess Of Loss Reinsurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding		705,000,000		675,000,000
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		12,005,000,000		12,638,000,000
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		245,084,000,000		252,314,000,000
Aircraft Residual Value Insurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding		$ 140,000,000		140,000,000
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Maximum period of liquidity claims (in years)		1 year
Net par amount outstanding		$ 11,599,000,000		12,238,000,000
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		3,198,000,000		3,367,000,000
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		8,401,000,000		8,871,000,000
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		5,377,000,000		5,392,000,000
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		4,678,000,000		4,818,000,000
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		10,681,000,000		11,224,000,000
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		201,337,000,000		209,392,000,000
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		2,666,000,000		2,761,000,000
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		3,091,000,000		3,261,000,000
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		6,646,000,000		7,140,000,000
United States [Member] | AAA [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		1,654,000,000		1,655,000,000
United States [Member] | AAA [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		866,000,000		877,000,000
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		4,967,000,000		4,966,000,000
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
Commitment to Provide Guarantees [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties		$ 7,000,000
Minimum [Member]
Schedule of Insured Financial Obligations [Line Items]
Probability of paying more claims than being reimbursed (as a percent)		50.00%
Maximum [Member] | Excess Of Loss Reinsurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Debt Service Outstanding		$ 1,000,000,000
PRASA (Puerto Rico Aqueduct and Sewer Authority) [Member]
Schedule of Insured Financial Obligations [Line Items]
Expected repayments of debt, percent		56.00%
Commonwealth of Puerto Rico - General Obligation Bonds [Member] | Subsequent Event [Member]
Schedule of Insured Financial Obligations [Line Items]
Certified Fiscal Plan, Commonwealth Debt Service, Period	6 years
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
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
Constitutionally Guaranteed [Member] | Commonwealth of Puerto Rico - General Obligation Bonds [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net par amount outstanding		$ 1,419,000,000		$ 1,419,000,000
Constitutionally Guaranteed [Member] | Commonwealth of Puerto Rico - General Obligation Bonds [Member] | Puerto Rico [Member] | Subsequent Event [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal Plan, Commonwealth Surplus Available for Debt Service			$ 6,700,000,000
Fiscal Plan, Commonwealth Contractual Debt Service			$ 15,500,000,000