Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums	$ 96	$ 101	$ 296	$ 285
Refundings	84	105	189	267
Terminations	3	21	15	65
Total Accelerations	87	126	204	332
Accretion of discount on net premiums receivable	3	4	11	11
Financial guaranty insurance net earned premiums	186	231	511	628
Other	0	0	1	0
Net	186	231	512	628
Variable Interest Entity, Primary Beneficiary [Member]
Financial Guarantee Insurance Premiums [Line Items]
Net	$ 3	$ 4	$ 11	$ 12