Outstanding Exposure - Narrative (Details) - USD ($)				12 Months Ended
	Oct. 23, 2018	Jul. 30, 2018	Apr. 20, 2018	Dec. 31, 2018	Feb. 12, 2019	Dec. 31, 2017	Sep. 15, 2015
Schedule of Insured Financial Obligations [Line Items]
Second-to pay insured par outstanding				$ 6,700,000,000		$ 6,600,000,000
Net Par Outstanding				241,802,000,000		264,952,000,000
Capital relief triple-X life insurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net debt service outstanding				763,000,000		675,000,000
Aircraft residual value insurance policies [Member]
Schedule of Insured Financial Obligations [Line Items]
Net debt service outstanding				218,000,000		140,000,000
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				11,137,000,000		12,638,000,000
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				$ 230,665,000,000		252,314,000,000
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Maximum period of liquidity claims (in years)				1 year
Loss mitigation securities held in investment portfolios excluding from net par				$ 1,900,000,000		2,000,000,000
Net Par Outstanding				10,160,000,000		12,238,000,000
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				2,731,000,000		3,367,000,000
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				7,429,000,000		8,871,000,000
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				4,767,000,000		4,966,000,000
Puerto Rico [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				4,767,000,000
Puerto Rico [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				4,800,000,000
VIRGIN ISLANDS, US
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				496,000,000
VIRGIN ISLANDS, US | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				222,000,000
VIRGIN ISLANDS, US | Internal Investment Grade [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				274,000,000
Commitment to Provide Guarantees [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties				$ 186,000,000
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 relief triple-X life insurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net debt service outstanding				$ 1,000,000,000
Commonwealth of Puerto Rico - General Obligation Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Commonwealth debt service, period	6 years
Commonwealth of Puerto Rico - General Obligation Bonds [Member] | COFINA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal Plan, Commonwealth, Sales and Use Tax, Rate, Allocations				46.35%
PRHTA (Transportation revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Commonwealth debt service, period			6 years
PRHTA (Highway revenue) [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Insured Financial Obligations, Secured By Pledges				$ 120,000,000
U.S. Justice Department and U.S. Environmental Protection Agency [Member] | PRASA [Member]
Schedule of Insured Financial Obligations [Line Items]
Settlement agreement, required spending threshold							$ 1,600,000,000
COFINA [Member] | COFINA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal Plan, Commonwealth, Sales and Use Tax, Rate, Allocations				53.65%
Constitutionally Guaranteed [Member] | Commonwealth of Puerto Rico - General Obligation Bonds [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Insured Financial Obligations, Fiscal Plan Budget Surplus	$ 17,000,000,000
Net Par Outstanding				$ 1,340,000,000		1,419,000,000
Constitutionally Guaranteed [Member] | Commonwealth of Puerto Rico - General Obligation Bonds [Member] | General Obligations Bonds Issues On or After March 2012 [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Insured Financial Obligations, Fiscal Plan Budget Surplus				369,000,000
Constitutionally Guaranteed [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Expected repayments of debt, percent		$ 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%
Constitutionally Guaranteed [Member] | Puerto Rico Public Buildings Authority [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				142,000,000		141,000,000
Public Corporations - Certain Revenues Potentially Subject to Clawback [Member] | PRHTA (Transportation revenue) [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				844,000,000		882,000,000
Public Corporations - Certain Revenues Potentially Subject to Clawback [Member] | PRHTA (Highway revenue) [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				475,000,000		495,000,000
Public Corporations - Certain Revenues Potentially Subject to Clawback [Member] | PRCCDA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				152,000,000		152,000,000
Public Corporations - Certain Revenues Potentially Subject to Clawback [Member] | PRIFA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				16,000,000		18,000,000
Other Public Corporations [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				848,000,000		853,000,000
Other Public Corporations [Member] | Puerto Rico Municipal Finance Authority [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				303,000,000		360,000,000
Other Public Corporations [Member] | PRASA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				373,000,000		373,000,000
Other Public Corporations [Member] | COFINA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				273,000,000		272,000,000
Other Public Corporations [Member] | U of PR [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding				$ 1,000,000		1,000,000
Closed Lien Senior Bonds [Member] | Subsequent Event [Member] | COFINA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Financial Guaranty Insurance, Plan of Adjustment, Closed Lien Senior Bonds Received					$ 152,000,000
Closed Lien Senior Current Interest Bonds [Member] | Subsequent Event [Member] | COFINA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Financial Guaranty Insurance, Plan of Adjustment, Closed Lien Senior Bonds Received					115,000,000
Closed Lien Senior Capital Appreciation Bonds [Member] | Subsequent Event [Member] | COFINA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Financial Guaranty Insurance, Plan of Adjustment, Closed Lien Senior Bonds Received					$ 37,000,000
Subordinated Bonds [Member] | Constitutionally Guaranteed [Member] | COFINA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Expected Repayments of Debt, Percent				60.00%
External Credit Rating, Non Investment Grade [Member] | Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member]
Schedule of Insured Financial Obligations [Line Items]
Second-to pay insured par outstanding				$ 111,000,000		$ 204,000,000