Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Retirement-Related Benefits
Defined benefit and contribution pension plans - cost	$ 672	$ 584
Nonpension postretirement plans - cost	44	52
Total	$ 717	$ 636
Year-to-year percent change, defined benefit and contribution pension plans cost (as a percent)	15.20%
Year-to-year percent change, nonpension postretirement plans cost (as a percent)	(14.50%)
Year-to-year percent change, total (as a percent)	12.80%