Contracts Accounted for as Insurance - Reconciliation of Net Expected Loss to be Paid and Expensed (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Guarantor Obligations [Line Items]
Net expected loss to be paid - financial guaranty insurance	$ (718)	$ (960)	$ (1,183)	$ (1,191)	$ (1,432)	$ (1,303)
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(971)		$ (1,143)
Net expected loss to be expensed	448
Variable Interest Entity, Primary Beneficiary [Member]
Guarantor Obligations [Line Items]
Net expected loss to be expensed	34
Financial Guarantee Insurance And Other Product Line [Member]
Guarantor Obligations [Line Items]
Net expected loss to be paid - financial guaranty insurance	661
Contra-paid, net	58
Salvage and subrogation recoverable, net, and other recoverable	699
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(970)
Net expected loss to be expensed	$ 448