Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums	$ 85	$ 92	$ 172	$ 180
Accelerations from refundings and terminations	20	39	46	91
Accretion of discount on net premiums receivable	5	4	9	8
Financial guaranty insurance net earned premiums	110	135	227	279
Non-financial guaranty net earned premiums	2	1	3	2
Net	112	136	230	281
Variable Interest Entity, Primary Beneficiary [Member]
Financial Guarantee Insurance Premiums [Line Items]
Net	$ 11	$ 3	$ 14	$ 6