Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums	$ 82	$ 87
Accelerations from refundings and terminations	15	26
Accretion of discount on net premiums receivable	5	4
Financial guaranty insurance net earned premiums	102	117
Specialty net earned premiums	1	1
Net	103	118
Variable Interest Entity, Primary Beneficiary [Member]
Financial Guarantee Insurance Premiums [Line Items]
Net	$ 1	$ 3