Insurance Premiums and Service Revenue Earned (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance Premiums and Service Revenue [Line Items]
Service revenue	$ 1,022	$ 973	$ 945
Insurance premiums written
Insurance Premiums and Service Revenue [Line Items]
Direct insurance premiums written	420	384	317
Assumed insurance premiums written	0	2	3
Insurance premiums, gross	420	386	320
Ceded insurance premiums written	(219)	(254)	(198)
Insurance premiums written, net	201	132	122
Service revenue	973	864	826
Insurance premiums and service revenue written and earned	1,174	996	948
Insurance premiums earned
Insurance Premiums and Service Revenue [Line Items]
Direct insurance premiums earned	414	364	318
Assumed insurance premiums earned	4	5	5
Insurance premiums, gross	418	369	323
Ceded insurance premiums earned	(197)	(188)	(141)
Insurance premiums earned, net	221	181	182
Service revenue	801	792	763
Insurance premiums and service revenue written and earned	$ 1,022	$ 973	$ 945