Contracts Accounted for as Insurance - Reconciliation of Net Expected Loss to be Paid and Expensed (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Guarantor Obligations [Line Items]
Net expected loss to be paid - financial guaranty insurance	$ (471)	$ (735)	$ (737)	$ (718)	$ (960)	$ (1,183)
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(974)		$ (1,012)
Net expected loss to be expensed	411
Variable Interest Entity, Primary Beneficiary
Guarantor Obligations [Line Items]
Net expected loss to be expensed	32
Financial Guarantee Insurance And Other Product Line
Guarantor Obligations [Line Items]
Net expected loss to be paid - financial guaranty insurance	418
Contra-paid, net	37
Salvage and subrogation recoverable, net, and other recoverable	927
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(971)
Net expected loss to be expensed	$ 411