Outstanding Exposure - Narrative (Details)			6 Months Ended
	Jul. 30, 2018 USD ($)	Jun. 29, 2018 USD ($)	Jun. 30, 2018 USD ($) judge	Jun. 07, 2018	Dec. 31, 2017 USD ($)	Sep. 15, 2015 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Loss mitigation securities			$ 1,900,000,000		$ 2,000,000,000
Total net exposure			$ 257,804,000,000		264,952,000,000
Number of federal judges | judge			5
Capital relieve triple-X life insurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Exposure			$ 738,000,000		675,000,000
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			11,984,000,000		12,638,000,000
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			245,820,000,000		252,314,000,000
Aircraft residual value insurance (RVI) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Exposure			$ 218,000,000		140,000,000
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Maximum period of liquidity claims (in years)			1 year
Total net exposure			$ 10,638,000,000		12,238,000,000
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			3,191,000,000		3,367,000,000
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			7,447,000,000		8,871,000,000
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			5,009,000,000		5,392,000,000
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			4,763,000,000		4,818,000,000
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			10,749,000,000		11,224,000,000
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			200,378,000,000		209,392,000,000
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			2,672,000,000		2,761,000,000
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			3,089,000,000		3,261,000,000
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			6,314,000,000		7,140,000,000
United States [Member] | AAA [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			1,778,000,000		1,655,000,000
United States [Member] | AAA [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			520,000,000		877,000,000
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			4,967,000,000		4,966,000,000
Fiscal Plan, Commonwealth, Sales and Use Tax, Rate				5.50%
Puerto Rico [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			5,000,000,000
U.S. Virgin Islands [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			498,000,000
U.S. Virgin Islands [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			224,000,000
U.S. Virgin Islands [Member] | Internal Investment Grade [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			274,000,000
Commitment to Provide Guarantees [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties			$ 25,000,000
Minimum [Member]
Schedule of Insured Financial Obligations [Line Items]
Probability of paying more claims than being reimbursed (as a percent)			50.00%
Maximum [Member] | Capital relieve triple-X life insurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Exposure			$ 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
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
COFINA (Puerto Rico Sales Tax Financing Corporation) [Member] | Commonwealth of Puerto Rico | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal Plan, Commonwealth, Sales and Use Tax, Rate, Allocations			46.35%
COFINA (Puerto Rico Sales Tax Financing Corporation) [Member] | COFINA (Puerto Rico Sales Tax Financing Corporation) [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal Plan, Commonwealth, Sales and Use Tax, Rate, Allocations			53.65%
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 | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			$ 1,419,000,000		$ 1,419,000,000
Fiscal Plan, Commonwealth Surplus Available for Debt Service		$ 6,700,000,000
Fiscal Plan, Commonwealth Contractual Debt Service		$ 15,500,000,000
Constitutionally Guaranteed [Member] | PREPA [Member] | Puerto Rico [Member] | Subsequent Event [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal Plan, Commonwealth Contractual Debt Service	$ 3,000,000,000
Fiscal Plan, Commonwealth Contractual Debt Service, New Securitization, Maturity	40 years
Fiscal Plan, Commonwealth Contractual Debt Service, New Securitization Percent of Par	67.00%
Fiscal Plan, Commonwealth Contractual Debt Service, New Securitization, Growth Bonds, Percent of Par	10.00%