Outstanding Exposure - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 23, 2018	Feb. 12, 2018	Dec. 31, 2016	Sep. 15, 2015
Schedule of Insured Financial Obligations [Line Items]
Net debt service outstanding	$ 401,118,000,000			$ 437,535,000,000
Net Par Outstanding	264,952,000,000			296,318,000,000
Excess Of Loss Reinsurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net debt service outstanding	675,000,000			390,000,000
3721 Aircraft [Member]
Schedule of Insured Financial Obligations [Line Items]
Net debt service outstanding	140,000,000
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net debt service outstanding	15,026,000,000			28,088,000,000
Net Par Outstanding	12,638,000,000			25,139,000,000
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net debt service outstanding	386,092,000,000			409,447,000,000
Net Par Outstanding	$ 252,314,000,000			271,179,000,000
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	$ 2,000,000,000			2,100,000,000
Net Par Outstanding	12,238,000,000			13,074,000,000
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	3,367,000,000			4,352,000,000
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	8,871,000,000			8,722,000,000
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net debt service outstanding	8,195,000,000
Net Par Outstanding	4,966,000,000			4,786,000,000
Puerto Rico [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	4,968,000,000
Puerto Rico [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	5,000,000,000
VIRGIN ISLANDS, US
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	498,000,000
VIRGIN ISLANDS, US | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	224,000,000
VIRGIN ISLANDS, US | Internal Investment Grade [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	$ 274,000,000
Minimum [Member]
Schedule of Insured Financial Obligations [Line Items]
Constant discount rate (as a percent)	4.00%
Probability of paying more claims than being reimbursed (as a percent)	50.00%
Maximum [Member]
Schedule of Insured Financial Obligations [Line Items]
Constant discount rate (as a percent)	4.50%
Maximum [Member] | Excess Of Loss Reinsurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net debt service outstanding	$ 1,000,000,000
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
Constitutionally Guaranteed [Member] | Commonwealth of Puerto Rico - General Obligation Bonds [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	1,419,000,000			1,476,000,000
Constitutionally Guaranteed [Member] | Puerto Rico Public Buildings Authority [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	141,000,000			169,000,000
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	882,000,000			918,000,000
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	495,000,000			350,000,000
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	18,000,000			18,000,000
Other Public Corporations [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	853,000,000			724,000,000
Other Public Corporations [Member] | Puerto Rico Municipal Finance Authority [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding	360,000,000			334,000,000
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	272,000,000			271,000,000
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
Subsequent Event [Member] | Commitment to Provide Guarantees [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties		$ 69,000,000
Subsequent Event [Member] | Constitutionally Guaranteed [Member] | Commonwealth of Puerto Rico - General Obligation Bonds [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal Plan, Commonwealth Contractual Debt Service			$ 17,500,000,000
Insured Financial Obligations, Fiscal Plan Budget Surplus			$ 2,800,000,000