Retirement-Related Benefits - Cost of Pension Plans (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Cost/(Income) of Pension Plans
Total defined benefit and contribution plans cost recognized in the Consolidated Statement of Earnings	$ 709	$ 659	$ 1,447	$ 1,323
U.S.
Cost/(Income) of Pension Plans
Total defined benefit and contribution plans cost recognized in the Consolidated Statement of Earnings	281	218	584	446
U.S. \| Pension Plans
Cost/(Income) of Pension Plans
Interest cost	$ 429	$ 478	$ 859	$ 957
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Expected return on plan assets	$ (675)	$ (753)	$ (1,351)	$ (1,507)
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Amortization of prior service costs/(credits)	$ 4	$ 4	$ 8	$ 8
Amortization of prior service costs/(credits) - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Recognized actuarial losses	$ 378	$ 331	$ 763	$ 668
Recognized actuarial losses - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 136	$ 60	$ 279	$ 127
Cost of defined contribution plans	145	158	305	319
Non-U.S.
Cost/(Income) of Pension Plans
Total defined benefit and contribution plans cost recognized in the Consolidated Statement of Earnings	428	442	863	877
Non-U.S. \| Pension Plans
Cost/(Income) of Pension Plans
Service cost	101	101	205	202
Interest cost	$ 211	$ 206	$ 426	$ 406
Interest cost - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Expected return on plan assets	$ (339)	$ (327)	$ (687)	$ (644)
Expected return on plan assets - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Amortization of prior service costs/(credits)	$ (21)	$ (24)	$ (42)	$ (48)
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Recognized actuarial losses	$ 353	$ 371	$ 712	$ 733
Recognized actuarial losses - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Curtailments and settlements	$ 6	$ 3	$ 6	$ 1
Curtailments and settlements - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Multi-employer plans	$ 10	$ 10	$ 20	$ 19
Other costs	$ 4	$ 5	$ 11	$ 10
Other costs - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 325	$ 344	$ 651	$ 679
Cost of defined contribution plans	$ 103	$ 97	$ 211	$ 198

Amount of cost for multi-employer defined benefit plans.

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Amount of cost for defined contribution plan.

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

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