Expected Loss to be Paid - Narrative (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) Curve Payment scenario	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) scenario	Dec. 31, 2018 USD ($)
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)	0.64%		1.94%
Total net exposure	$ 230,898		$ 236,807
Net expected loss to be paid after recoveries for R&W	660	$ 963	$ 737	$ 1,183
Economic loss development after recoveries for R&W	3	2
Changes in discount rates	$ (31)	4
Additional loss recovery assumption, recovery period	5 years
Loss Recovery Assumption, Additional Increase in Recovery Projection, Percent	30.00%
Loss Recovery Assumption, Additional Increase in Recovery Projection, Economic Benefit	$ 55
Loss Recovery Assumption, Additional Decrease in Recovery Projection, Percent	10.00%
Loss Recovery Assumption, Additional Decrease in Recovery Projection, Economic Loss	$ 55
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	1.39%		2.45%
Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	$ 4,270		$ 4,270
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	3.90%		3.20%
RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	$ 3,393		$ 3,546
Net expected loss to be paid after recoveries for R&W	104	237	146	293
Economic loss development after recoveries for R&W	63	65
Future net R&W benefit	$ 106		53
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	$ 4,347		4,744
Net expected loss to be paid after recoveries for R&W	37	29	37	26
Economic loss development after recoveries for R&W	(1)	(2)
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	221,370		226,746
Net expected loss to be paid after recoveries for R&W	519	697	554	864
Economic loss development after recoveries for R&W	(59)	(61)
Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	172,795		175,487
Net expected loss to be paid after recoveries for R&W	493	666	531	$ 832
Net expected credit for estimated future recoveries of claims paid	911		819
Economic loss development after recoveries for R&W	(56)	(62)
Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	48,575
Net expected loss to be paid after recoveries for R&W	26		23
Public Finance Stockton Pension Oblgiation Bonds [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	107
BIG [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	8,238		8,506
BIG [Member] | Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	4,300
BIG [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	1,580		1,618
BIG [Member] | Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	151		179
BIG [Member] | Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	40
BIG [Member] | Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	80
BIG [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	6,467		6,669
BIG [Member] | Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	5,630		5,771
BIG [Member] | Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Total net exposure	837
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	110		$ 166
Economic loss development after recoveries for R&W	(59)	(31)
Changes in discount rates	$ 25
Number of delinquent payments | Payment	2
Projected loss assumptions, CDR, plateau rate, projection period	36 months
Projected loss assumptions, Final CPR, 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
Performing or projected to reperform, projection period	36 months
Period from plateau to intermediate conditional default rate (in months)	12 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 CPR, Period for voluntary prepayments to continue	3 years 3 months
Default from delinquentor rate, term	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	$ 46
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	$ 49
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	$ 6		$ 20
Economic loss development after recoveries for R&W	$ (4)	(34)
Period from plateau to intermediate conditional default rate (in months)	28 months
Number of scenarios weighted in estimating expected losses | scenario			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] | Most Stressful [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] | Most Stressful [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	$ 6
Second Lien [Member] | Lease Stressful [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] | Lease Stressful [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	$ 6
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	$ (13)		$ (4)
Economic loss development after recoveries for R&W	$ 8	$ 2