Retirement-Related Benefits - Nonpension Postretirement Cost (Details) - Nonpension Postretirement Plans - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
U.S.
Nonpension postretirement plan cost
Service cost	$ 2	$ 2	$ 5	$ 7
Interest cost	$ 16	$ 26	$ 49	$ 77
Interest cost - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Expected return on plan assets - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Amortization of prior service costs/(credits)	$ 1	$ 1	$ 3	$ 3
Amortization of prior service costs/(credits) - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Recognized actuarial losses	$ 13	$ 7	$ 39	$ 22
Recognized actuarial losses - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Curtailments and settlements - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 32	$ 36	$ 96	$ 109
Non-U.S.
Nonpension postretirement plan cost
Service cost	1	1	4	4
Interest cost	$ 8	$ 8	$ 25	$ 26
Interest cost - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Expected return on plan assets	$ (1)	$ (1)	$ (2)	$ (3)
Expected return on plan assets - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Amortization of prior service costs/(credits)	$ 0	$ 0	$ 0	$ 0
Amortization of prior service costs/(credits) - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Recognized actuarial losses	$ 4	$ 5	$ 12	$ 16
Recognized actuarial losses - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Curtailments and settlements			$ 0	$ 0
Curtailments and settlements - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 13	$ 14	$ 38	$ 43