Contingencies: (Details) (USD $) In Millions, unless otherwise specified		0 Months Ended	3 Months Ended	9 Months Ended		1 Months Ended			0 Months Ended
	Sep. 30, 2013 countries	Oct. 12, 2012 Litigation in United Kingdom regarding defined benefit plans (C Plan)	Sep. 30, 2012 Litigation in United Kingdom regarding defined benefit plans (C Plan)	Sep. 30, 2012 Litigation in United Kingdom regarding defined benefit plans (C Plan)	Sep. 30, 2013 Former IBM UK Defined Benefit Plan Participants IBM United Kingdom Limited claims	Jul. 25, 2013 Civil enforcement action with the SEC IBM Korea, LG IBM, IBM (China) Investment Company Limited and IBM Global Services (China) Co., Ltd.	Mar. 31, 2011 Civil enforcement action with the SEC IBM Korea, LG IBM, IBM (China) Investment Company Limited and IBM Global Services (China) Co., Ltd.	Jul. 18, 2012 State of Indiana	Jul. 10, 2013 SCO Group Inc. claims
Contingencies:
Clients presence in number of countries	170
Loss Contingencies [Line Items]
Amount of settlement to be paid/(received)							$ 10	$ (52)
Claims pending					290
Additional pre-tax retirement-related obligation recorded			162	162
Retirement age of employee under the C Plan		60 years
Retirement age of employee under the C Plan before modifications		63 years
Profits gained as a result of the conduct alleged in the SEC's complaint							5.3
Prejudgment interest							2.7
Civil penalty							2
Period for which reports are to be submitted to SEC and court on certain matters, including those relating to compliance with the FCPA						2 years
Number of breaches of contract and copyright claims filed									10
Number of breaches of contract and copyright claims dismissed									7
Number of tortious interference claims dismissed									1
Income tax examination - Brazil, total potential liability	$ 700