Retirement-Related Benefits - Cost of Pension Plans (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Cost/(Income) of Pension Plans
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	$ 653	$ 596	$ 1,325	$ 1,179
U.S. \| Pension Plans
Cost/(Income) of Pension Plans
Interest cost	$ 277	$ 375	$ 555	$ 750
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Expected return on plan assets	$ (451)	$ (542)	$ (901)	$ (1,084)
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Amortization of prior service costs/(credits)	$ 4	$ 4	$ 8	$ 8
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Recognized actuarial losses	$ 249	$ 207	$ 498	$ 415
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Curtailments and settlements - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Other costs/(credits) - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 80	$ 44	$ 160	$ 89
Cost of defined contribution plans	151	154	302	309
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	231	198	462	398
Non-U.S. \| Pension Plans
Cost/(Income) of Pension Plans
Service cost	95	95	187	190
Interest cost	$ 112	$ 137	$ 223	$ 266
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Expected return on plan assets	$ (291)	$ (311)	$ (581)	$ (620)
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Amortization of prior service costs/(credits)	$ (4)	$ (5)	$ (6)	$ (9)
Amortization of prior service costs/(credits) - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Recognized actuarial losses	$ 370	$ 340	$ 745	$ 682
Recognized actuarial losses - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Curtailments and settlements	$ 16	$ 13	$ 34	$ 21
Curtailments and settlements - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Multi-employer plans	$ 7	$ 7	$ 15	$ 14
Other costs/(credits)	$ 3	$ 9	$ 15	$ 14
Other costs/(credits) - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 308	$ 284	$ 630	$ 559
Cost of defined contribution plans	114	113	234	223
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	$ 422	$ 397	$ 864	$ 782

Amount of cost for multi-employer defined benefit plans.

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