Retirement-Related Benefits - Nonpension Postretirement Contributions (Details) - Nonpension Postretirement Plans - U.S. - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016
Retirement-Related Benefits
Contributions by employer - Noncash	$ 259	$ 200
Contributions by employer - Cash		$ 40

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