Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Retirement Benefits [Abstract]
Defined benefit pension and defined contribution pension plans — cost	$ 233	$ 323
Nonpension postretirement plans — cost	32	30
Total	$ 266	$ 353
Year-to-year percent change, Defined benefit pension and defined contribution plans — cost (as a percent)	(27.70%)
Year-to-year percent change, Nonpension postretirement plans — cost (as a percent)	6.20%
Year-to-year percent change, Total (as a percent)	(24.80%)