Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Retirement-Related Benefits
Defined benefit and contribution pension plans - cost	$ 584	$ 437
Nonpension postretirement plans - cost	52	54
Total	$ 636	$ 491
Year-to-year percent change, defined benefit and contribution pension plans cost (as a percent)	33.70%
Year-to-year percent change, nonpension postretirement plans cost (as a percent)	(4.10%)
Year-to-year percent change, total (as a percent)	29.50%