Contracts Accounted for as Insurance - Reconciliation of Net Expected Loss to be Paid and Expensed (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Guarantor Obligations [Line Items]
Net expected loss to be paid after recoveries for R&W	$ (1,244)	$ (1,198)	$ (1,337)	$ (1,391)
Contra-paid, net	47	37
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(1,119)	(1,047)
Net expected loss to be expensed	457
Variable Interest Entity, Primary Beneficiary [Member]
Guarantor Obligations [Line Items]
Net expected loss to be expensed	60
Financial Guarantee Insurance And Other Product Line [Member]
Guarantor Obligations [Line Items]
Net expected loss to be paid after recoveries for R&W	1,141
Contra-paid, net	47
Salvage and subrogation recoverable, net of reinsurance	385
Loss and LAE reserve - financial guaranty insurance contracts, net of reinsurance	(1,118)
Other recoverable (payable)	2
Net expected loss to be expensed	457
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	$ 58	$ 64