Retirement-Related Benefits - Nonpension Postretirement Cost (Details) - Nonpension Postretirement Plans - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
U.S.
Nonpension postretirement plan cost
Service cost	$ 2	$ 2	$ 4	$ 4
Interest cost	$ 16	$ 26	$ 32	$ 51
Interest cost - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Expected return on plan assets - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Amortization of prior service costs/(credits)	$ 1	$ 1	$ 2	$ 2
Amortization of prior service costs/(credits) - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Recognized actuarial losses	$ 13	$ 7	$ 26	$ 15
Recognized actuarial losses - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Curtailments and settlements - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 32	$ 36	$ 64	$ 72
Non-U.S.
Nonpension postretirement plan cost
Service cost	1	1	2	3
Interest cost	$ 8	$ 8	$ 16	$ 18
Interest cost - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Expected return on plan assets	$ (1)	$ (1)	$ (2)	$ (2)
Expected return on plan assets - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Amortization of prior service costs/(credits)	$ 0	$ 0	$ 0	$ 0
Amortization of prior service costs/(credits) - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Recognized actuarial losses	$ 4	$ 5	$ 8	$ 11
Recognized actuarial losses - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Curtailments and settlements	$ 0		$ 0	$ 0
Curtailments and settlements - income statement location	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember	ibm:OtherIncomeExpenseMember
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 13	$ 14	$ 25	$ 29

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

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