Retirement-Related Benefits - Nonpension Postretirement Cost (Details) - Nonpension Postretirement Plans - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
U.S.
Retirement-related plans cost
Service cost	$ 3	$ 3	$ 5	$ 6
Interest cost	$ 36	$ 33	$ 73	$ 66
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)	$ (2)	$ (1)	$ (4)
Amortization of prior service costs/(credits) - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Recognized actuarial losses	$ 0	$ 2	$ 0	$ 5
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 (income)/cost recognized for defined benefit plans	$ 38	$ 37	$ 77	$ 74
Non-U.S.
Retirement-related plans cost
Service cost	1	1	3	3
Interest cost	$ 12	$ 11	$ 25	$ 24
Interest cost - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Expected return on plan assets	$ (1)	$ (1)	$ (3)	$ (3)
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	$ 3	$ 1	$ 5	$ 3
Recognized actuarial losses - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Curtailments and settlements		$ 0	$ 0	$ 0
Curtailments and settlements - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Total net periodic (income)/cost recognized for defined benefit plans	$ 15	$ 12	$ 30	$ 26

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

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