Outstanding Exposure - Narrative (Details) - USD ($) $ in Millions			3 Months Ended	12 Months Ended
	Oct. 23, 2018	Jun. 29, 2018	Mar. 31, 2019	Dec. 31, 2018	May 03, 2019	Feb. 12, 2019	Jan. 14, 2019	Aug. 01, 2018	Sep. 15, 2015
Schedule of Insured Financial Obligations [Line Items]
Second-to-pay insured par outstanding			$ 6,500	$ 6,700
Loss mitigation securities			1,900	1,900
Total net exposure			237,325	241,802
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			11,302	11,137
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			226,023	230,665
Aircraft residual value insurance (RVI) [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Exposure			$ 239	218
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Maximum period of liquidity claims (in years)			1 year
Total net exposure			$ 9,626	10,160
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			2,568	2,731
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			7,058	7,429
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			4,610	4,618
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			4,064	4,270
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			10,337	9,944
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			181,408	186,562
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			2,239	2,387
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			2,481	2,632
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			6,057	6,388
United States [Member] | AAA [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			1,510	1,533
United States [Member] | AAA [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			412	413
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			4,494	4,767
Puerto Rico [Member] | Closed Lien Senior Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Financial Guaranty Insurance, Plan of Adjustment, Closed Lien Senior Bonds Received						$ 152
Puerto Rico [Member] | Closed Lien Senior Current Interest Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Financial Guaranty Insurance, Plan of Adjustment, Closed Lien Senior Bonds Received			115
Puerto Rico [Member] | Closed Lien Senior Capital Appreciation Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Financial Guaranty Insurance, Plan of Adjustment, Closed Lien Senior Bonds Received			37
Puerto Rico [Member] | COFINA Exchange Senior Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Financial Guaranty Insurance, Plan of Adjustment, Closed Lien Senior Bonds Received						$ 139
Puerto Rico [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			4,500
U.S. Virgin Islands [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			496
U.S. Virgin Islands [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			222
U.S. Virgin Islands [Member] | Internal Investment Grade [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			274
Commitment to Provide Guarantees [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties			$ 147
Commitment to Provide Guarantees [Member] | Public Finance [Member] | Subsequent Event [Member]
Schedule of Insured Financial Obligations [Line Items]
Outstanding commitments to provide guaranties					$ 136
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
Commonwealth of Puerto Rico [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
PRHTA [Member]
Schedule of Insured Financial Obligations [Line Items]
Certified Fiscal Plan, Commonwealth Debt Service, Period		6 years
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			1,340	$ 1,340
Fiscal Plan, Commonwealth Surplus Available for Debt Service	$ 17,000
Constitutionally Guaranteed [Member] | Commonwealth of Puerto Rico [Member] | General Obligations Bonds Issues On or After March 2012 [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Fiscal Plan, Commonwealth General Obligation Bonds, Litigation Amount							$ 6,000
Fiscal Plan, Commonwealth Surplus Available for Debt Service			369
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%
Other Public Corporations [Member] | PRASA (Puerto Rico Aqueduct and Sewer Authority) [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			373	$ 373
Other Public Corporations [Member] | MFA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			303	303
Other Public Corporations [Member] | COFINA (Puerto Rico Sales Tax Financing Corporation) [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			0
Other Public Corporations [Member] | COFINA (Puerto Rico Sales Tax Financing Corporation) [Member] | Puerto Rico [Member] | Closed Lien Senior Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure				273
Other Public Corporations [Member] | PREPA [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			848	848
Fiscal Plan, Commonwealth Contractual Debt Service Restructuring, Minimum Bondholder Support Threshold, Percent								67.00%
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			$ 108	$ 111