Retirement Benefits- Projected Benefit Obligations and Accumulated Benefit Obligations in Excess of Plan Assets (Details) - Accumulated Benefit Obligation In Excess Of Plan Assets [Member] - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
ABO	$ 23,514	$ 21,727
Fair value of net plan assets	$ 20,344	$ 18,590