Outstanding Insurance Exposure - Puerto Rico Net Par Outstanding (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 230,898	$ 236,807
Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	4,270	4,270
Constitutionally Guaranteed [Member] | Puerto Rico [Member] | Commonwealth of Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1,253	1,253
Constitutionally Guaranteed [Member] | Puerto Rico [Member] | Puerto Rico Public Buildings Authority [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	140	140
Public Corporations, Certain Revenue Potentially Subject to Clawback [Member] | Puerto Rico [Member] | PRHTA (Transportation revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	811	811
Public Corporations, Certain Revenue Potentially Subject to Clawback [Member] | Puerto Rico [Member] | PRHTA (Highway revenue) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	454	454
Public Corporations, Certain Revenue Potentially Subject to Clawback [Member] | Puerto Rico [Member] | PRCCDA (Puerto Rico Convention Center District Authority) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	152	152
Public Corporations, Certain Revenue Potentially Subject to Clawback [Member] | Puerto Rico [Member] | PRIFA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	16	16
Other Public Corporations [Member] | Puerto Rico [Member] | PREPA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	822	822
Other Public Corporations [Member] | Puerto Rico [Member] | PRASA (Puerto Rico Aqueduct and Sewer Authority) [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	373	373
Other Public Corporations [Member] | Puerto Rico [Member] | MFA [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	248	248
Other Public Corporations [Member] | Puerto Rico [Member] | University of Puerto Rico [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	1	$ 1
Vintage General Obligation Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	669
Insured Financial Obligations, Net Principal Claims Paid	383
Insured Financial Obligations, Net Interest Claims Paid, Amount	$ 165
Insured Financial Obligations, Base Recovery As Percent of Pre-Petition Claims	74.90%
General Obligation Bonds, 2011, Series D, E and PIB [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 5
Insured Financial Obligations, Net Principal Claims Paid	6
Insured Financial Obligations, Net Interest Claims Paid, Amount	$ 1
Insured Financial Obligations, Base Recovery As Percent of Pre-Petition Claims	73.80%
General Obligation Bonds, 2011, Series C [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 210
Insured Financial Obligations, Net Principal Claims Paid	0
Insured Financial Obligations, Net Interest Claims Paid, Amount	$ 48
Insured Financial Obligations, Base Recovery As Percent of Pre-Petition Claims	70.40%
General Obligation Bonds, 2012 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 369
Insured Financial Obligations, Net Principal Claims Paid	0
Insured Financial Obligations, Net Interest Claims Paid, Amount	$ 72
Insured Financial Obligations, Base Recovery As Percent of Pre-Petition Claims	69.90%
General Obligation Bonds, 2014 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 0
Insured Financial Obligations, Net Principal Claims Paid	0
Insured Financial Obligations, Net Interest Claims Paid, Amount	$ 0
Insured Financial Obligations, Base Recovery As Percent of Pre-Petition Claims	65.40%
Vintage Public Buildings Authority Bonds [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 140
Insured Financial Obligations, Net Principal Claims Paid	32
Insured Financial Obligations, Net Interest Claims Paid, Amount	$ 27
Insured Financial Obligations, Base Recovery As Percent of Pre-Petition Claims	77.60%
Public Buildings Authority Bonds, 2011 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 0
Insured Financial Obligations, Net Principal Claims Paid	0
Insured Financial Obligations, Net Interest Claims Paid, Amount	$ 0
Insured Financial Obligations, Base Recovery As Percent of Pre-Petition Claims	76.80%
Public Buildings Authority Bonds, 2012 [Member]
Schedule of Insured Financial Obligations [Line Items]
Total net exposure	$ 0
Insured Financial Obligations, Net Principal Claims Paid	0
Insured Financial Obligations, Net Interest Claims Paid, Amount	$ 0
Insured Financial Obligations, Base Recovery As Percent of Pre-Petition Claims	72.20%