Retirement-Related Benefits - Nonpension Postretirement Cost (Details) - Nonpension Postretirement Benefit Plans - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
U.S.
Post retirement plan cost
Service cost	$ 2	$ 3	$ 7	$ 8
Interest cost	$ 26	$ 36	$ 77	$ 109
Interest cost - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Expected return on plan assets - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Amortization of prior service costs/(credits)	$ 1	$ (1)	$ 3	$ (2)
Amortization of prior service costs/(credits) - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Recognized actuarial losses	$ 7	$ 0	$ 22	$ 0
Recognized actuarial losses - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Curtailments and settlements - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 36	$ 38	$ 109	$ 115
Non-U.S.
Post retirement plan cost
Service cost	1	1	4	4
Interest cost	$ 8	$ 12	$ 26	$ 37
Interest cost - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Expected return on plan assets	$ (1)	$ (1)	$ (3)	$ (4)
Expected return on plan assets - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Amortization of prior service costs/(credits)	$ 0	$ 0	$ 0	$ 0
Amortization of prior service costs/(credits) - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Recognized actuarial losses	$ 5	$ 3	$ 16	$ 8
Recognized actuarial losses - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Curtailments and settlements			$ 0	$ 0
Curtailments and settlements - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 14	$ 15	$ 43	$ 45

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

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