Retirement-Related Benefits - Nonpension Postretirement Cost (Details) - Nonpension Postretirement Plans - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
U.S.
Retirement-related plans cost
Service cost	$ 3	$ 3
Interest cost	$ 36	$ 33
Interest cost - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Amortization of prior service costs/(credits)	$ (1)	$ (2)
Amortization of prior service costs/(credits) - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Recognized actuarial losses	$ 1	$ 2
Recognized actuarial losses - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Total net periodic (income)/cost recognized for defined benefit plans	$ 39	$ 37
Non-U.S.
Retirement-related plans cost
Service cost	1	1
Interest cost	$ 12	$ 13
Interest cost - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Expected return on plan assets	$ (1)	$ (1)
Expected return on plan assets - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Amortization of prior service costs/(credits)	$ 0	$ 0
Amortization of prior service costs/(credits) - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Recognized actuarial losses	$ 3	$ 1
Recognized actuarial losses - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Curtailments and settlements	$ 0	$ 0
Curtailments and settlements - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Total net periodic (income)/cost recognized for defined benefit plans	$ 15	$ 14

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

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Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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