Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums	$ 95	$ 96	$ 275	$ 296
Accelerations from refundings and terminations	40	87	131	204
Accretion of discount on net premiums receivable	6	3	14	11
Financial guaranty insurance net earned premiums	141	186	420	511
Non-financial guaranty net earned premiums	1	0	3	1
Net	142	186	423	512
Variable Interest Entity, Primary Beneficiary [Member]
Financial Guarantee Insurance Premiums [Line Items]
Net	$ 3	$ 3	$ 9	$ 11