Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums	$ 97	$ 93	$ 200	$ 184
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Refunded Amount	49	83	105	162
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Terminated Amount	10	34	12	44
Accelerations:	59	117	117	206
Accretion of discount on net premiums receivable	5	4	8	7
Premiums Earned, Net, Financial Guarantee Insurance Contracts	161	214	325	397
Other	1	0	1	0
Net	162	214	326	397
Variable Interest Entity, Primary Beneficiary [Member]
Financial Guarantee Insurance Premiums [Line Items]
Net	$ 4	$ 3	$ 8	$ 8