Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Retirement-Related Benefits
Defined benefit and contribution pension plans - cost	$ 705	$ 592	$ 2,152	$ 1,915
Nonpension postretirement plans - cost	49	61	149	182
Total	$ 754	$ 653	$ 2,300	$ 2,097
Year-to-year percent change, defined benefit and contribution pension plans cost (as a percent)	19.00%		12.30%
Year-to-year percent change, nonpension postretirement plans cost (as a percent)	(19.50%)		(18.30%)
Year-to-year percent change, total (as a percent)	15.50%		9.70%