Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums									$ 331	$ 367	$ 385
Accelerations from refundings and terminations									122	159	286
Accretion of discount on net premiums receivable									17	18	17
Financial guaranty insurance net earned premiums									470	544	688
Specialty net earned premiums									6	4	2
Net earned premiums	$ 123	$ 123	$ 112	$ 118	$ 125	$ 142	$ 136	$ 145	476	548	690
Variable Interest Entity, Primary Beneficiary [Member]
Financial Guarantee Insurance Premiums [Line Items]
Net earned premiums									$ 18	$ 12	$ 15