Retirement-Related Benefits - Nonpension Postretirement Cost (Details) - Nonpension Postretirement Benefit Plans - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
U.S.
Post retirement plan cost
Service cost	$ 2	$ 3
Interest cost	$ 26	$ 36
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Expected return on plan assets - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Amortization of prior service costs/(credits)	$ 1	$ (1)
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Recognized actuarial losses	$ 7	$ 1
Recognized actuarial losses - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Curtailments and settlements - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 36	$ 39
Non-U.S.
Post retirement plan cost
Service cost	1	1
Interest cost	$ 10	$ 12
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Expected return on plan assets	$ (1)	$ (1)
Expected return on plan assets - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Amortization of prior service costs/(credits)	$ 0	$ 0
Amortization of prior service costs/(credits) - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Recognized actuarial losses	$ 6	$ 3
Recognized actuarial losses - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Curtailments and settlements	$ 0	$ 0
Curtailments and settlements - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 16	$ 15

Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan.

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