Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Retirement-Related Benefits
Defined benefit and contribution pension plans - cost	$ 610	$ 459	$ 1,789	$ 1,337
Nonpension postretirement plans - cost	50	53	151	160
Total	$ 660	$ 512	$ 1,941	$ 1,497
Year-to-year percent change, defined benefit and contribution pension plans cost (as a percent)	32.90%		33.80%
Year-to-year percent change, nonpension postretirement plans cost (as a percent)	(5.90%)		(5.20%)
Year-to-year percent change, total (as a percent)	28.90%		29.60%