Retirement-Related Benefits - Cost of Pension Plans (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2012	Sep. 30, 2018	Sep. 30, 2017
Cost/(Income) of Pension Plans
Total defined benefit and contribution pension plans cost recognized in the Consolidated Statement of Earnings	$ 705	$ 592		$ 2,152	$ 1,915
U.S.
Cost/(Income) of Pension Plans
Total defined benefit and contribution pension plans cost recognized in the Consolidated Statement of Earnings	292	219		876	665
U.S. \| Pension Plans
Cost/(Income) of Pension Plans
Interest cost	$ 430	$ 478		$ 1,289	$ 1,435
Interest cost - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense		ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Expected return on plan assets	$ (675)	$ (753)		$ (2,026)	$ (2,260)
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Amortization of prior service costs/(credits)	$ 4	$ 4		$ 12	$ 12
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Recognized actuarial losses	$ 381	$ 334		$ 1,144	$ 1,003
Recognized actuarial losses - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense		ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 140	$ 63		$ 419	$ 190
Cost of defined contribution plans	152	156		457	475
Non-U.S.
Cost/(Income) of Pension Plans
Total defined benefit and contribution pension plans cost recognized in the Consolidated Statement of Earnings	413	373		1,276	1,250
Non-U.S. \| Pension Plans
Cost/(Income) of Pension Plans
Service cost	104	106		309	308
Interest cost	$ 204	$ 215		$ 630	$ 622
Interest cost - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense		ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Expected return on plan assets	$ (330)	$ (340)		$ (1,017)	$ (985)
Expected return on plan assets - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense		ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Amortization of prior service costs/(credits)	$ (20)	$ (25)		$ (62)	$ (73)
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Recognized actuarial losses	$ 346	$ 387		$ 1,057	$ 1,120
Recognized actuarial losses - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense		ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Curtailments and settlements	$ 2	$ 2		$ 7	$ 3
Curtailments and settlements - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense		ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Multi-employer plans	$ 9	$ 11		$ 29	$ 30
Other costs	$ 5	$ (85)		$ 16	$ (76)
Other costs - income statement location	ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense		ibm:OtherIncomeAndExpense	ibm:OtherIncomeAndExpense
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 318	$ 271		$ 969	$ 950
Cost of defined contribution plans	$ 95	102		$ 306	301
Non-U.S. \| Pension Plans \| Litigation in United Kingdom regarding defined benefit plans \| SG&A Expense
Cost/(Income) of Pension Plans
Other costs		$ (91)	$ 162		$ (91)

Amount of cost for multi-employer defined benefit plans.

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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 for defined contribution plan.

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Amount of cost (reversal of cost) for pension benefits. Excludes other postretirement benefits.

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