Retirement Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Allocated Service Cost	$ 59	$ 47	$ 55
Defined Contribution Plan, Cost	8	8	7
Other Postretirement Benefits Cost (Reversal of Cost)	$ 3	$ 3	$ 4