Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Retirement-Related Benefits
Defined benefit and contribution pension plans - cost	$ 653	$ 596	$ 1,325	$ 1,179
Nonpension postretirement plans - cost	45	50	89	102
Total	$ 698	$ 645	$ 1,414	$ 1,281
Year-to-year percent change, defined benefit and contribution pension plans cost (as a percent)	9.60%		12.40%
Year-to-year percent change, nonpension postretirement plans cost (as a percent)	(10.40%)		(12.50%)
Year-to-year percent change, total (as a percent)	8.10%		10.40%