Retirement-Related Benefits - Cost of Pension Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cost/(Income) of Pension Plans
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	$ 672	$ 584
U.S. | Pension Plans
Cost/(Income) of Pension Plans
Interest cost	$ 277	$ 375
Interest cost - income statement location	Other (income) and expense	Other (income) and expense
Expected return on plan assets	$ (451)	$ (542)
Expected return on plan assets - income statement location	Other (income) and expense	Other (income) and expense
Amortization of prior service costs/(credits)	$ 4	$ 4
Amortization of prior service costs/(credits) - income statement location	Other (income) and expense	Other (income) and expense
Recognized actuarial losses	$ 249	$ 207
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	$ 80	$ 44
Cost of defined contribution plans	151	155
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	231	199
Non-U.S. | Pension Plans
Cost/(Income) of Pension Plans
Service cost	92	95
Interest cost	$ 111	$ 129
Interest cost - income statement location	Other (income) and expense	Other (income) and expense
Expected return on plan assets	$ (291)	$ (309)
Expected return on plan assets - income statement location	Other (income) and expense	Other (income) and expense
Amortization of prior service costs/(credits)	$ (2)	$ (5)
Amortization of prior service costs/(credits) - income statement location	Other (income) and expense	Other (income) and expense
Recognized actuarial losses	$ 375	$ 342
Recognized actuarial losses - income statement location	Other (income) and expense	Other (income) and expense
Curtailments and settlements	$ 17	$ 8
Curtailments and settlements - income statement location	Other (income) and expense	Other (income) and expense
Multi-employer plans	$ 8	$ 7
Other costs/(credits)	$ 11	$ 5
Other costs/(credits) - income statement location	Other (income) and expense	Other (income) and expense
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 322	$ 274
Cost of defined contribution plans	119	110
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	$ 441	$ 385