Retirement Benefits- Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure
Defined Benefit Plan, Service Cost	$ 1	$ 0	$ 0
Interest cost	2	1	1
Expected return on plan assets	(7)	(7)	(6)
Prior service credit amortization	14	1	1
Actuarial loss amortization	0	0	(1)
Net periodic benefit cost	$ 10	$ (5)	$ (5)