Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Retirement-Related Benefits
Defined benefit and contribution pension plans - cost	$ 647	$ 610	$ 1,972	$ 1,789
Nonpension postretirement plans - cost	45	50	134	151
Total	$ 692	$ 660	$ 2,106	$ 1,941
Year-to-year percent change, defined benefit and contribution pension plans cost (as a percent)	6.00%		10.20%
Year-to-year percent change, nonpension postretirement plans cost (as a percent)	(10.50%)		(11.80%)
Year-to-year percent change, total (as a percent)	4.80%		8.50%