Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums	$ 88	$ 103
Refundings	46	56
Terminations	6	2
Total Accelerations	52	58
Accretion of discount on net premiums receivable	4	3
Financial guaranty insurance net earned premiums	144	164
Non-financial guaranty net earned premiums	1	0
Net	145	164
Variable Interest Entity, Primary Beneficiary [Member]
Financial Guarantee Insurance Premiums [Line Items]
Net	$ 3	$ 4