Retirement-Related Benefits - All Retirement Plans Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Retirement-Related Benefits
Defined benefit and contribution pension plans - cost	$ 437	$ 738
Nonpension postretirement plans - cost	54	51
Total	$ 491	$ 788
Year-to-year percent change, defined benefit and contribution pension plans cost (as a percent)	(40.80%)
Year-to-year percent change, nonpension postretirement plans cost (as a percent)	6.40%
Year-to-year percent change, total (as a percent)	(37.70%)