Outstanding Exposure - Narrative (Details) - USD ($) $ in Millions			6 Months Ended	12 Months Ended
	Jun. 05, 2019	May 09, 2019	Jun. 30, 2019	Dec. 31, 2018	Jun. 27, 2019	Jun. 16, 2019	May 21, 2019	Feb. 12, 2019	Jan. 14, 2019	Sep. 15, 2015
Schedule of Insured Financial Obligations [Line Items]
Second-to-pay insured par outstanding			$ 6,600	$ 6,700
Loss mitigation securities			1,500	1,900
Total net exposure			235,367	241,802
Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			10,342	11,137
Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			225,025	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			$ 8,863	10,160
BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			1,954	2,731
BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			6,909	7,429
AAA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			4,454	4,618
United States [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			3,835	4,270
United States [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			9,549	9,944
United States [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			180,537	186,562
United States [Member] | BIG [Member] | RMBS [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			1,723	2,387
United States [Member] | BIG [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			1,913	2,632
United States [Member] | BIG [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			6,025	6,388
United States [Member] | AAA [Member] | Structured Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			1,428	1,533
United States [Member] | AAA [Member] | Public Finance [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			385	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			495
U.S. Virgin Islands [Member] | BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			221
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			$ 295
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			$ 949
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		$ 3,000
Fiscal plan, commonwealth general obligation bonds, litigation amount						$ 35,000
Vintage GO bondholders, expected receivable of outstanding claims, percent						64.30%
Vintage GO bondholders, additional expected receivable of outstanding claims, percent						25.10%
Expected receivable of outstanding claims, additional amount from Commonwealth if it outperforms fiscal plan, percent						16.80%
Fiscal plan, commonwealth surplus available for debt service		$ 13,700
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				$ 220
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] | Puerto Rico Public Buildings Authority [Member] | Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure			$ 142	$ 142
Vintage GO bondholders, expected receivable of outstanding claims, percent			72.60%
Vintage GO bondholders, additional expected receivable of outstanding claims, percent						89.40%
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			$ 107	$ 111