Retirement-Related Benefits - Cost of Pension Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Cost/(Income) of Pension Plans
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	$ 647	$ 610	$ 1,972	$ 1,789
U.S. | Pension Plans
Cost/(Income) of Pension Plans
Interest cost	$ 277	$ 375	$ 832	$ 1,126
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	$ (451)	$ (542)	$ (1,352)	$ (1,627)
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)	$ 4	$ 4	$ 12	$ 12
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	$ 249	$ 207	$ 747	$ 622
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
Other costs/(credits) - 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	$ 80	$ 44	$ 239	$ 133
Cost of defined contribution plans	152	148	455	457
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	232	192	694	590
Non-U.S. | Pension Plans
Cost/(Income) of Pension Plans
Service cost	94	99	281	289
Interest cost	$ 109	$ 142	$ 332	$ 408
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	$ (286)	$ (323)	$ (867)	$ (943)
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)	$ (2)	$ (5)	$ (8)	$ (14)
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	$ 365	$ 360	$ 1,110	$ 1,041
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	$ 13	$ 21	$ 46	$ 42
Curtailments and settlements - income statement location	Other (income) and expense	Other (income) and expense	Other (income) and expense	Other (income) and expense
Multi-employer plans	$ 3	$ 7	$ 17	$ 22
Other costs/(credits)	$ 7	$ 6	$ 21	$ 20
Other costs/(credits) - 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	$ 303	$ 307	$ 933	$ 865
Cost of defined contribution plans	112	111	345	334
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	$ 415	$ 418	$ 1,278	$ 1,200