Contracts Accounted for as Insurance - Reconciliation of Net Expected Loss to be Paid and Expensed (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Guarantor Obligations [Line Items]
Net expected loss to be paid - financial guaranty insurance	$ (660)	$ (737)	$ (963)	$ (1,183)
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(1,015)	$ (1,012)
Net expected loss to be expensed	427
Variable Interest Entity, Primary Beneficiary [Member]
Guarantor Obligations [Line Items]
Net expected loss to be expensed	33
Financial Guarantee Insurance And Other Product Line [Member]
Guarantor Obligations [Line Items]
Net expected loss to be paid - financial guaranty insurance	607
Contra-paid, net	44
Salvage and subrogation recoverable, net, and other recoverable	789
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(1,013)
Net expected loss to be expensed	$ 427