Retirement-Related Benefits - Cost of Pension Plans (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Cost/(Income) of Pension Plans
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	$ 596	$ 441	$ 1,179	$ 878
U.S. \| Pension Plans
Cost/(Income) of Pension Plans
Interest cost	$ 375	$ 469	$ 750	$ 941
Interest cost - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Expected return on plan assets	$ (542)	$ (650)	$ (1,084)	$ (1,300)
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Amortization of prior service costs/(credits)	$ 4	$ 4	$ 8	$ 8
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Recognized actuarial losses	$ 207	$ 139	$ 415	$ 279
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Curtailments and settlements - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Other costs - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 44	$ (37)	$ 89	$ (71)
Cost of defined contribution plans	154	140	309	289
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	198	103	398	218
Non-U.S. \| Pension Plans
Cost/(Income) of Pension Plans
Service cost	95	93	190	185
Interest cost	$ 137	$ 206	$ 266	$ 413
Interest cost - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Expected return on plan assets	$ (311)	$ (396)	$ (620)	$ (795)
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Amortization of prior service costs/(credits)	$ (5)	$ (7)	$ (9)	$ (13)
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Recognized actuarial losses	$ 340	$ 312	$ 682	$ 626
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Curtailments and settlements	$ 13	$ 3	$ 21	$ 4
Curtailments and settlements - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Multi-employer plans	$ 7	$ 8	$ 14	$ 17
Other costs/(credits)	$ 9	$ 6	$ 14	$ 11
Other costs - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 284	$ 225	$ 559	$ 449
Cost of defined contribution plans	113	114	223	212
Total defined benefit and contribution pension plans cost recognized in the Consolidated Income Statement	$ 397	$ 339	$ 782	$ 661

Amount of cost for multi-employer defined benefit plans.

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