Retirement Benefits - Balance Sheet locations of Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	$ 31	$ 26
Pension obligations	4,247	3,988
Non-pension postretirement benefit obligations	0	0
Unfunded status (net benefit obligation less plan assets)	4,278	4,014
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Other current liabilities	41	33
Pension obligations	0	0
Non-pension postretirement benefit obligations	2,076	1,928
Unfunded status (net benefit obligation less plan assets)	$ 2,117	$ 1,961