Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums	$ 85	$ 81	$ 250	$ 253
Accelerations from refundings and terminations	17	37	64	83
Accretion of discount on net premiums receivable	5	4	15	13
Financial guaranty insurance net earned premiums	107	122	329	349
Specialty net earned premiums	0	1	2	4
Net	(107)	(123)	(331)	(353)
Variable Interest Entity, Primary Beneficiary
Financial Guarantee Insurance Premiums [Line Items]
Net	$ 2	$ 2	$ 4	$ 16