Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2025 |
Jun. 30, 2024 |
Jun. 30, 2025 |
Jun. 30, 2024 |
|
| Retirement Benefits [Abstract] | ||||
| Defined benefit pension and defined contribution pension plans — cost | $ 248 | $ 320 | $ 482 | $ 643 |
| Nonpension postretirement plans — cost | 32 | 30 | 65 | 60 |
| Total | $ 281 | $ 350 | $ 546 | $ 704 |
| Year-to-year percent change, Defined benefit pension and defined contribution plans — cost (as a percent) | (22.50%) | (25.10%) | ||
| Year-to-year percent change, Nonpension postretirement plans — cost (as a percent) | 8.30% | 7.20% | ||
| Year-to-year percent change, Total (as a percent) | (19.90%) | (22.40%) | ||