Contracts Accounted for as Insurance - Reconciliation of Net Expected Loss to be Paid and Expensed (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Guarantor Obligations [Line Items]
Net expected loss to be paid (recovered) after recoveries for R&W	$ (1,303)	$ (1,198)	$ (1,391)
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(1,400)	(1,047)
Other recoverable (payable)	4	$ (1)
Net expected loss to be expensed (present value)	442
Variable Interest Entity, Primary Beneficiary [Member]
Guarantor Obligations [Line Items]
Net expected loss to be expensed (present value)	52
Financial Guarantee Insurance And Other Product Line [Member]
Guarantor Obligations [Line Items]
Net expected loss to be paid (recovered) after recoveries for R&W	1,226
Contra-paid, net	59
Salvage and subrogation recoverable, net of reinsurance	552
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(1,399)
Other recoverable (payable)	4
Net expected loss to be expensed (present value)	$ 442