Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums									$ 367	$ 385	$ 381
Accelerations from refundings and terminations									159	286	469
Accretion of discount on net premiums receivable									18	17	14
Financial guaranty insurance net earned premiums									544	688	864
Non-financial guaranty net earned premiums									4	2	0
Net earned premiums	$ 125	$ 142	$ 136	$ 145	$ 178	$ 186	$ 162	$ 164	548	690	864
Variable Interest Entity, Primary Beneficiary [Member]
Financial Guarantee Insurance Premiums [Line Items]
Net earned premiums									$ 12	$ 15	$ 16