Expected Loss to be Paid - Narrative (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) Curve Payment scenario	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) scenario	Dec. 31, 2017 USD ($) scenario	Nov. 26, 2012 USD ($)
Schedule of Expected Losses to be Paid [Line Items]
Period of insured credit performance of guaranteed obligations (in some cases over)	30 years
Discount factor (as a percent)	2.46%		2.74%
Total net exposure	$ 237,325		$ 241,802
Net expected loss to be paid after recoveries for R&W	963	$ 1,298	$ 1,183	$ 1,303
Economic loss development after recoveries for R&W	$ (2)	(24)
Minimum [Member]
Schedule of Expected Losses to be Paid [Line Items]
Risk free discount rate	0.00%		0.00%
Maximum [Member]
Schedule of Expected Losses to be Paid [Line Items]
Risk free discount rate	2.87%		3.06%
Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	$ 4,494		$ 4,767
Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for representations and warranties, percent	3.20%		2.70%
RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	$ 4,064		$ 4,270
Net expected loss to be paid after recoveries for R&W	237	219	293	73
Economic loss development after recoveries for R&W	(65)	16
Future net R&W benefit	$ 19		5
RMBS [Member] | United States [Member] | Minimum [Member] | HELOCs [Member]
Schedule of Expected Losses to be Paid [Line Items]
Final CPR	2.50%
HELOCs [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial period for which borrower can pay only interest payments	10 years
Extended period for which borrow can pay only interest payments	5 years
Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	$ 5,487		5,683
Net expected loss to be paid after recoveries for R&W	29	29	26	27
Economic loss development after recoveries for R&W	2	2
Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	1,800
Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	1,100
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	226,023		230,665
Net expected loss to be paid after recoveries for R&W	697	1,050	864	1,203
Economic loss development after recoveries for R&W	61	(42)
Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	181,408		186,562
Net expected loss to be paid after recoveries for R&W	666	1,007	832	1,157
Net expected credit for estimated future recoveries of claims paid	652		586
Economic loss development after recoveries for R&W	62	(39)
Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	44,615		44,103
Net expected loss to be paid after recoveries for R&W	31	43	32	$ 46
Economic loss development after recoveries for R&W	(1)	(3)
Public Finance Stockton Pension Oblgiation Bonds [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	110
Non-Infrastructure Public Finance [Member] | Spain [Member] | Sovereign and Sub Sovereign [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	420
Non-Infrastructure Public Finance [Member] | Portugal [Member] | Sovereign and Sub Sovereign [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	71
BIG [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	9,626		10,160
BIG [Member] | Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	4,500
BIG [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	2,239		2,387
BIG [Member] | Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	244		259
BIG [Member] | Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	85
BIG [Member] | Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	96
BIG [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	7,058		7,429
BIG [Member] | Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	6,057		6,388
BIG [Member] | Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	1,001		1,041
First Lien [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	209		$ 243
Economic loss development after recoveries for R&W	$ 31	(24)
Number of delinquent payments | Payment	2
Projected loss assumptions, CDR, plateau rate, projection period	36 months
Projected loss assumptions, Final CDR, Period for voluntary prepayments to continue	12 months
Intermediate conditional default rate (as a percent)	5.00%
Number of scenarios weighted in estimating expected losses | scenario	5			5
First Lien [Member] | Base Scenario [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, CDR, plateau rate, projection period	36 months
Period from plateau to intermediate conditional default rate (in months)	12 months
Period of constant intermediate conditional default rate (in months)	36 months
Intermediate conditional default rate as a percentage of plateau conditional default rate	20.00%
Final conditional default rate as a percentage of plateau conditional default rate	5.00%
Projected loss assumptions, Final CDR, Period for voluntary prepayments to continue	4 years 3 months
Default from delinquentor rate, term	36 months
Guarantor obligations, default period currently performing	36 months
Projected loss assumptions, loss severity, subsequent period	18 months
Estimated loss severity rate, one through six months (as a percent)	18 months
Loss severity (as a percent)	40.00%
Projected loss assumptions, period to reach final loss severity rate	2 years 6 months
Final CPR	15.00%		15.00%
First Lien [Member] | More Stressful Environment [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Period from plateau to intermediate conditional default rate (in months)	15 months
Projected loss assumptions, period to reach final loss severity rate	9 years
Projected loss assumptions, increase (decrease) in expected loss to be paid, net	$ 54
First Lien [Member] | Least Stressful Environment [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, CDR, plateau rate, projection period	30 months
Period from plateau to intermediate conditional default rate (in months)	9 months
Projected loss assumptions, increase (decrease) in expected loss to be paid, net	$ 38
Decrease in the plateau period used to calculate potential change in loss estimate (in months)	6 months
Second Lien [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	$ 28		$ 50
Economic loss development after recoveries for R&W	$ 34	8
Period from plateau to intermediate conditional default rate (in months)	28 months
Number of scenarios weighted in estimating expected losses | scenario	5		5
Period of loan default estimate	6 months
Number of preceding months average liquidation rates used to estimate loan default rate	6 months
Projected loss assumptions, period of consistent conditional default rate	6 months
Stress period (in months)	34 months
Loss recovery assumption (as a percent)	2.00%		2.00%
Number of conditional default rate curves modeled in estimating losses | Curve	5
Monthly delinquency threshold	6 months
Second Lien [Member] | RMBS [Member] | United States [Member] | Home Equity Line of Credit and Closed-end Mortgage [Member]
Schedule of Expected Losses to be Paid [Line Items]
Final CPR	15.00%		15.00%
Second Lien [Member] | Base Scenario [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Stress period (in months)	34 months
Second Lien [Member] | Base Scenario One [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, CDR, plateau rate, projection period	8 months
Period from plateau to intermediate conditional default rate (in months)	31 months
Stress period (in months)	39 months
Increase in conditional default rate ramp down period	3 months
Second Lien [Member] | Base Scenario One [Member] | RMBS [Member] | United States [Member] | HELOCs [Member]
Schedule of Expected Losses to be Paid [Line Items]
Change in estimate for increased conditional default rate plateau period	$ 8
Second Lien [Member] | Based Scenario Two [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Stress period (in months)	29 months
Period of constant conditional default rate (in months)	4 months
Change in estimate for decreased prepayment rate, Percent	10.00%
Decreased conditional default rate ramp down period	25 months
Second Lien [Member] | Based Scenario Two [Member] | RMBS [Member] | United States [Member] | HELOCs [Member]
Schedule of Expected Losses to be Paid [Line Items]
Change in estimate for decreased conditional default rate ramp down period	$ 9
Southern District of Mississippi Vs Madison County, Mississippi [Member] | Parkway East [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	18
MBIA UK Insurance Limited [Member] | Europe [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	$ 195
Second Lien [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Period from plateau to intermediate conditional default rate (in months)	28 months
Period of constant conditional default rate (in months)	6 months
Credit derivatives [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	$ (6)		$ (2)
Economic loss development after recoveries for R&W	$ (2)	$ 7
ACA 2005-2 Collateralized Debt Obligations [Member] | CIFG Holding Inc. [Member] | Total [Member] | Credit derivatives [Member] | CIFG Holdings Inc. vs JP Morgan Securities LLC [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W					$ 400
Libertas II Collateralized Debt Obligations [Member] | CIFG Holding Inc. [Member] | Total [Member] | Credit derivatives [Member] | CIFG Holdings Inc. vs JP Morgan Securities LLC [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W					$ 325