Retirement Benefits - Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Worldwide OPEB
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)
Service cost	$ 12	$ 12
Interest cost	32	43
Expected return on assets	0	0
Amortization of prior service costs/(credits)	(3)	(4)
Net remeasurement (gain)/loss	0	58
Defined Benefit Plan, Other Cost (Credit)	0	(1)
Settlements and curtailments	0	(2)
Net periodic benefit cost/(income)	41	106
Pension Plan | Global Redesign
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)
Net periodic benefit cost/(income)	38	24
Foreign Plan | Pension Plan
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)
Service cost	144	131
Interest cost	99	133
Expected return on assets	(285)	(267)
Amortization of prior service costs/(credits)	5	9
Net remeasurement (gain)/loss	(484)	(80)
Defined Benefit Plan, Other Cost (Credit)	37	24
Settlements and curtailments	1	1
Net periodic benefit cost/(income)	(483)	(49)
UNITED STATES | Pension Plan
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)
Service cost	138	130
Interest cost	223	323
Expected return on assets	(702)	(699)
Amortization of prior service costs/(credits)	1	1
Net remeasurement (gain)/loss	423	0
Defined Benefit Plan, Other Cost (Credit)	2	10
Settlements and curtailments	39	0
Net periodic benefit cost/(income)	$ 124	$ (235)