Retirement Benefits- Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure
Defined Benefit Plan, Service Cost	$ 1	$ 1	$ 1
Interest cost	3	3	2
Expected return on plan assets	(9)	(7)	(7)
Prior service credit amortization	18	24	14
Net periodic benefit cost	$ 13	$ 21	$ 10