Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Retirement Benefits [Abstract]
Defined benefit pension and defined contribution plans — cost	$ 233	$ 3,024	$ 714	$ 3,667
Nonpension postretirement plans — cost	33	30	97	90
Total	$ 265	$ 3,053	$ 812	$ 3,757
Year-to-year percent change, Defined benefit pension and defined contribution plans — cost (as a percent)	(92.30%)		(80.50%)
Year-to-year percent change, Nonpension postretirement plans — cost (as a percent)	10.20%		8.20%
Year-to-year percent change, Total (as a percent)	(91.30%)		(78.40%)