Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums	$ 103	$ 91
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Refunded Amount	56	79
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Terminated Amount	2	10
Accelerations:	58	89
Accretion of discount on net premiums receivable	3	3
Premiums Earned, Net, Financial Guarantee Insurance Contracts	164	183
Other	0	0
Net	164	183
Variable Interest Entity, Primary Beneficiary [Member]
Financial Guarantee Insurance Premiums [Line Items]
Net	$ 4	$ 5