Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Retirement-Related Benefits
Defined benefit and contribution pension plans - cost	$ 738	$ 664
Nonpension postretirement plans - cost	51	61
Total	$ 788	$ 725
Year-to-year percent change, defined benefit and contribution pension plans cost (as a percent)	11.10%
Year-to-year percent change, nonpension postretirement plans cost (as a percent)	(17.20%)
Year-to-year percent change, total (as a percent)	8.70%