Outstanding Exposure - Narrative (Details) $ in Millions				3 Months Ended	9 Months Ended
	Oct. 23, 2018 USD ($)	Jul. 30, 2018 USD ($)	Apr. 20, 2018	Sep. 30, 2018 USD ($) judge	Sep. 30, 2018 USD ($) judge	Sep. 20, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 15, 2015 USD ($)
Schedule of Insured Financial Obligations [Line Items]
Loss mitigation securities				$ 1,900	$ 1,900		$ 2,000
Total net exposure				$ 246,940	$ 246,940		264,952
Number of federal judges | judge				5	5
Capital relieve triple-X life insurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Exposure				$ 766	$ 766		675
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				11,787	11,787		12,638
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				235,153	235,153		252,314
Aircraft residual value insurance (RVI) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Exposure				218	$ 218		140
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Maximum period of liquidity claims (in years)					1 year
Total net exposure				10,293	$ 10,293		12,238
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				3,051	3,051		3,367
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				7,242	7,242		8,871
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				4,819	4,819		5,392
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				4,566	4,566		4,818
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				10,611	10,611		11,224
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				190,418	190,418		209,392
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				2,560	2,560		2,761
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				2,949	2,949		3,261
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				6,171	6,171		7,140
United States [Member] | AAA [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				1,691	1,691		1,655
United States [Member] | AAA [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				428	428		877
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				4,767	4,767		4,966
Puerto Rico [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				4,800	4,800
U.S. Virgin Islands [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				496	496
U.S. Virgin Islands [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				222	222
U.S. Virgin Islands [Member] | Internal Investment Grade [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				274	274
Commitment to Provide Guarantees [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties				$ 440	$ 440
Minimum [Member]
Schedule of Insured Financial Obligations [Line Items]
Probability of paying more claims than being reimbursed (as a percent)				50.00%	50.00%
Maximum [Member] | Capital relieve triple-X life insurance [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Exposure				$ 1,000	$ 1,000
Commonwealth of Puerto Rico [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	120
PRHTA [Member]
Schedule of Insured Financial Obligations [Line Items]
Certified Fiscal Plan, Commonwealth Debt Service, Period			6 years
COFINA (Puerto Rico Sales Tax Financing Corporation) [Member] | Commonwealth of Puerto Rico [Member] | 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
Constitutionally Guaranteed [Member] | Commonwealth of Puerto Rico [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				$ 1,341	$ 1,341		$ 1,419
Constitutionally Guaranteed [Member] | Commonwealth of Puerto Rico [Member] | Puerto Rico [Member] | Subsequent Event [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal Plan, Commonwealth Surplus Available for Debt Service	$ 17,000
Constitutionally Guaranteed [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						5.50%
Fiscal Plan, Commonwealth Contractual Debt Service						$ 10,000
Constitutionally Guaranteed [Member] | COFINA (Puerto Rico Sales Tax Financing Corporation) [Member] | Puerto Rico [Member] | Subordinated Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Expected repayments of debt, percent					60.00%
Constitutionally Guaranteed [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal Plan, Commonwealth Contractual Debt Service		$ 3,000
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%