Contracts Accounted for as Insurance - Reconciliation of Net Expected Loss to be Paid and Expensed (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Guarantor Obligations [Line Items]
Net expected loss to be paid after recoveries for R&W	$ (1,191)	$ (1,432)	$ (1,303)	$ (1,292)	$ (1,297)	$ (1,198)
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(1,113)		$ (1,400)
Net expected loss to be expensed	525
Variable Interest Entity, Primary Beneficiary [Member]
Guarantor Obligations [Line Items]
Net expected loss to be expensed	45
Financial Guarantee Insurance And Other Product Line [Member]
Guarantor Obligations [Line Items]
Net expected loss to be paid after recoveries for R&W	1,112
Contra-paid, net	73
Salvage and subrogation recoverable, net of reinsurance	449
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(1,112)
Other recoverable (payable)	3
Net expected loss to be expensed	$ 525