Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Retirement-Related Benefits
Defined benefit and contribution pension plans - cost	$ 459	$ 705	$ 1,337	$ 2,152
Nonpension postretirement plans - cost	53	49	160	149
Total	$ 512	$ 754	$ 1,497	$ 2,300
Year-to-year percent change, defined benefit and contribution pension plans cost (as a percent)	(34.90%)		(37.80%)
Year-to-year percent change, nonpension postretirement plans cost (as a percent)	8.60%		7.40%
Year-to-year percent change, total (as a percent)	(32.10%)		(34.90%)