Retirement-Related Benefits - Nonpension Postretirement Cost (Details) - Nonpension Postretirement Benefit Plans - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
U.S.
Nonpension postretirement plan cost
Service cost	$ 2	$ 2
Interest cost	$ 16	$ 26
Interest cost - income statement location	Other (income) and expense	Other (income) and expense
Amortization of prior service costs/(credits)	$ 1	$ 1
Amortization of prior service costs/(credits) - income statement location	Other (income) and expense	Other (income) and expense
Recognized actuarial losses	$ 13	$ 7
Recognized actuarial losses - income statement location	Other (income) and expense	Other (income) and expense
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 32	$ 36
Non-U.S.
Nonpension postretirement plan cost
Service cost	1	1
Interest cost	$ 8	$ 10
Interest cost - income statement location	Other (income) and expense	Other (income) and expense
Expected return on plan assets	$ (1)	$ (1)
Expected return on plan assets - income statement location	Other (income) and expense	Other (income) and expense
Amortization of prior service costs/(credits)	$ 0	$ 0
Amortization of prior service costs/(credits) - income statement location	Other (income) and expense	Other (income) and expense
Recognized actuarial losses	$ 4	$ 6
Recognized actuarial losses - income statement location	Other (income) and expense	Other (income) and expense
Curtailments and settlements		$ 0
Curtailments and settlements - income statement location		Other (income) and expense
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 12	$ 16