Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Retirement Benefits [Abstract]
Defined benefit pension and defined contribution pension plans — cost	$ 323	$ 281
Nonpension postretirement plans — cost	30	32
Total	$ 353	$ 314
Year-to-year percent change, Defined benefit pension and defined contribution plans — cost (as a percent)	14.80%
Year-to-year percent change, Nonpension postretirement plans — cost (as a percent)	(6.00%)
Year-to-year percent change, Total (as a percent)	12.70%