Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums	$ 81	$ 95	$ 253	$ 275
Accelerations from refundings and terminations	37	40	83	131
Accretion of discount on net premiums receivable	4	6	13	14
Financial guaranty insurance net earned premiums	122	141	349	420
Non-financial guaranty net earned premiums	1	1	4	3
Net	123	142	353	423
Variable Interest Entity, Primary Beneficiary [Member]
Financial Guarantee Insurance Premiums [Line Items]
Net	$ 2	$ 3	$ 16	$ 9