Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Retirement-Related Benefits
Defined benefit and contribution pension plans - cost	$ 709	$ 659	$ 1,447	$ 1,323
Nonpension postretirement plans - cost	49	60	100	121
Total	$ 758	$ 719	$ 1,546	$ 1,444
Year-to-year percent change, defined benefit and contribution pension plans cost (as a percent)	7.50%		9.30%
Year-to-year percent change, nonpension postretirement plans cost (as a percent)	(18.30%)		(17.80%)
Year-to-year percent change, total (as a percent)	5.40%		7.10%