Retirement-Related Benefits - Nonpension Postretirement Cost (Details) - Nonpension Postretirement Plans - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
U.S.
Retirement-related plans cost
Service cost	$ 3	$ 4
Interest cost	33	38
Amortization of prior service costs/(credits)	(2)	(2)
Recognized actuarial losses	2	5
Total net periodic pension / nonpension (income)/cost of defined benefit plans	37	46
Non-U.S.
Retirement-related plans cost
Service cost	1	1
Interest cost	13	14
Expected return on plan assets	(1)	(2)
Amortization of prior service costs/(credits)	0	0
Recognized actuarial losses	1	2
Curtailments and settlements	0	0
Total net periodic pension / nonpension (income)/cost of defined benefit plans	$ 14	$ 16