Segments: (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Revenue	$ 23,720	$ 24,747	$ 72,052	$ 75,203
Pre-tax income	4,812	5,074	12,562	14,071
Global Technology Services
Segment Reporting Information [Line Items]
Revenue	9,494	9,922	28,634	29,952
Global Business Services
Segment Reporting Information [Line Items]
Revenue	4,558	4,542	13,649	13,846
Software
Segment Reporting Information [Line Items]
Revenue	5,798	5,763	17,792	17,533
Systems and Technology
Segment Reporting Information [Line Items]
Revenue	3,247	3,895	10,111	11,903
Global Financing
Segment Reporting Information [Line Items]
Revenue	502	472	1,488	1,478
Total Segments
Segment Reporting Information [Line Items]
Revenue	23,599	24,594	71,674	74,713
Business Segments
Segment Reporting Information [Line Items]
Revenue	25,461	26,570	77,383	80,561
Pre-tax income	5,579	5,389	15,003	15,637
Business Segments | Global Technology Services
Segment Reporting Information [Line Items]
Revenue	9,755	10,206	29,435	30,821
Pre-tax income	1,895	1,697	4,994	4,934
Revenue year-to-year change (as a percent)	(4.40%)		(4.50%)
Pre-tax income year-to-year change (as a percent)	11.70%		1.20%
Pre-tax income margin (as a percent)	19.40%	16.60%	17.00%	16.00%
Business Segments | Global Business Services
Segment Reporting Information [Line Items]
Revenue	4,735	4,717	14,194	14,384
Pre-tax income	948	738	2,274	2,142
Revenue year-to-year change (as a percent)	0.40%		(1.30%)
Pre-tax income year-to-year change (as a percent)	28.40%		6.10%
Pre-tax income margin (as a percent)	20.00%	15.60%	16.00%	14.90%
Business Segments | Software
Segment Reporting Information [Line Items]
Revenue	6,542	6,606	20,105	19,992
Pre-tax income	2,410	2,355	6,867	6,793
Revenue year-to-year change (as a percent)	(1.00%)		0.60%
Pre-tax income year-to-year change (as a percent)	2.30%		1.10%
Pre-tax income margin (as a percent)	36.80%	35.60%	34.20%	34.00%
Business Segments | Systems and Technology
Segment Reporting Information [Line Items]
Revenue	3,415	4,076	10,533	12,394
Pre-tax income	(167)	124	(713)	253
Revenue year-to-year change (as a percent)	(16.20%)		(15.00%)
Pre-tax income margin (as a percent)	(4.90%)	3.00%	(6.80%)	2.00%
Business Segments | Global Financing
Segment Reporting Information [Line Items]
Revenue	1,015	963	3,116	2,970
Pre-tax income	494	476	1,582	1,516
Revenue year-to-year change (as a percent)	5.30%		4.90%
Pre-tax income year-to-year change (as a percent)	3.80%		4.40%
Pre-tax income margin (as a percent)	48.70%	49.40%	50.80%	51.00%
Business Segments | Total Segments
Segment Reporting Information [Line Items]
Revenue	25,461	26,570	77,383	80,561
Pre-tax income	5,579	5,389	15,003	15,637
Revenue year-to-year change (as a percent)	(4.20%)		(3.90%)
Pre-tax income year-to-year change (as a percent)	3.50%		(4.10%)
Pre-tax income margin (as a percent)	21.90%	20.30%	19.40%	19.40%
Internal transactions
Segment Reporting Information [Line Items]
Revenue	(1,863)	(1,976)	(5,710)	(5,848)
Pre-tax income	(280)	(322)	(998)	(949)
Internal transactions | Global Technology Services
Segment Reporting Information [Line Items]
Revenue	262	285	801	869
Internal transactions | Global Business Services
Segment Reporting Information [Line Items]
Revenue	177	175	545	538
Internal transactions | Software
Segment Reporting Information [Line Items]
Revenue	744	843	2,312	2,459
Internal transactions | Systems and Technology
Segment Reporting Information [Line Items]
Revenue	168	181	423	491
Internal transactions | Global Financing
Segment Reporting Information [Line Items]
Revenue	512	491	1,628	1,492
Internal transactions | Total Segments
Segment Reporting Information [Line Items]
Revenue	$ 1,863	$ 1,976	$ 5,710	$ 5,848