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Restructuring - Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) position | Dec. 31, 2016 USD ($) position | Dec. 31, 2015 USD ($) position
Restructuring Cost and Reserve [Line Items] |  |  |
Restructuring costs | $ 927 | $ 1,069 | $ 1,110
Cumulative restructuring costs incurred to date since program inception | $ 13,500 |  |
Number of positions elimination since inception of program | position | 43,350 |  |
Expected remaining costs related to restructuring program activities | $ 500 |  |
Percentage estimate of cumulative pretax costs that will result in cash outlays (primarily from employee separation expense) (as percent) | 66.67% |  |
Percentage estimate of cumulative pretax costs that will be non-cash (primarily from accelerated depreciation of facilities) (as percent) | 33.33% |  |
Number of positions eliminated | position | 2,450 | 2,625 | 3,770
Asset abandonment, shut-down and other related costs |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Restructuring costs | $ 267 | $ 409 | $ 550
Facility Closing |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Pretax losses resulting from sales of facilities and related assets | $ 6 | $ 151 | $ 117

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