Contracts Accounted for as Insurance - Reconciliation of Net Expected Loss to be Paid and Expensed (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Jan. 10, 2017	Dec. 31, 2016	Sep. 30, 2016	Jul. 01, 2016	Jun. 30, 2016	Dec. 31, 2015
Guarantor Obligations [Line Items]
Net expected loss to be paid after recoveries for R&W	$ (1,292)	$ (1,297)	$ (21)	$ (1,198)	$ (1,090)	$ (22)	$ (1,326)	$ (1,391)
Contra-paid, net	51			37
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(1,287)			(1,047)
Net expected loss to be expensed	449
Variable Interest Entity, Primary Beneficiary [Member]
Guarantor Obligations [Line Items]
Net expected loss to be expensed	55
Financial Guarantee Insurance And Other Product Line [Member]
Guarantor Obligations [Line Items]
Net expected loss to be paid after recoveries for R&W	1,205
Contra-paid, net	51
Salvage and subrogation recoverable, net of reinsurance	476
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(1,286)
Other recoverable (payable)	3
Net expected loss to be expensed	449
Financial Guarantee Insurance And Other Product Line [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Guarantor Obligations [Line Items]
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	$ 54			$ 64