Retirement-Related Benefits - Nonpension Postretirement Cost (Details) - Nonpension Postretirement Plans - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
U.S.
Retirement-related plans cost
Service cost	$ 4	$ 4	$ 11	$ 13
Interest cost	38	41	115	124
Amortization of prior service costs/(credits)	(2)	(2)	(6)	(6)
Recognized actuarial losses	5	5	15	15
Total net periodic pension / nonpension (income)/cost of defined benefit plans	45	49	135	146
Non-US
Retirement-related plans cost
Service cost	1	1	4	4
Interest cost	14	15	43	37
Expected return on plan assets	(2)	(2)	(6)	(5)
Amortization of prior service costs/(credits)	0	(1)	0	(4)
Recognized actuarial losses	2	2	5	7
Curtailments and settlements	0	0	0	0
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 16	$ 16	$ 47	$ 39