Retirement Benefits - Benefit Obligations and Plan Assets in Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	$ 47	$ 31
Pension obligations	4,503	4,247
Non-pension postretirement benefit obligations	0	0
Unfunded status (net benefit obligation less plan assets)	4,550	4,278
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	42	41
Pension obligations	0	0
Non-pension postretirement benefit obligations	2,008	2,076
Unfunded status (net benefit obligation less plan assets)	$ 2,050	$ 2,117