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, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 21,727	$ 19,656
Fair value of net plan assets	$ 18,590	$ 16,678