Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums	$ 83	$ 85	$ 165	$ 172
Accelerations from refundings and terminations	32	20	47	46
Accretion of discount on net premiums receivable	5	5	10	9
Financial guaranty insurance net earned premiums	120	110	222	227
Specialty net earned premiums	1	2	2	3
Net	(121)	(112)	(224)	(230)
Variable Interest Entity, Primary Beneficiary [Member]
Financial Guarantee Insurance Premiums [Line Items]
Net	$ 1	$ 11	$ 2	$ 14