Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums	$ 81	$ 85	$ 242	$ 250
Accelerations from refundings and terminations	8	17	39	64
Accretion of discount on net premiums receivable	13	5	24	15
Financial guaranty insurance net earned premiums	102	107	305	329
Specialty net earned premiums	0	0	2	2
Net	102	107	307	331
Variable Interest Entity, Primary Beneficiary
Financial Guarantee Insurance Premiums [Line Items]
Net	$ 1	$ 2	$ 3	$ 4