Retirement-Related Benefits - Cost of Pension Plans (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Cost/(Income) of Pension Plans
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	$ 584	$ 437
U.S. \| Pension Plans
Cost/(Income) of Pension Plans
Interest cost	$ 375	$ 472
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Expected return on plan assets	$ (542)	$ (650)
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Amortization of prior service costs/(credits)	$ 4	$ 4
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Recognized actuarial losses	$ 207	$ 140
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Curtailments and settlements - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Other costs - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 44	$ (34)
Cost of defined contribution plans	155	149
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	199	115
Non-U.S. \| Pension Plans
Cost/(Income) of Pension Plans
Service cost	95	93
Interest cost	$ 129	$ 208
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Expected return on plan assets	$ (309)	$ (399)
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Amortization of prior service costs/(credits)	$ (5)	$ (6)
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Recognized actuarial losses	$ 342	$ 314
Recognized actuarial losses - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Curtailments and settlements	$ 8	$ 1
Curtailments and settlements - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Multi-employer plans	$ 7	$ 9
Other costs/(credits)	$ 5	$ 5
Other costs - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 274	$ 223
Cost of defined contribution plans	110	98
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	$ 385	$ 322

Amount of cost for multi-employer defined benefit plans.

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Amount of cost (reversal of cost) for pension benefits. Excludes other postretirement benefits.

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