| INCOME TAXES (Tables)
 | 12 Months Ended | 
| Dec. 31, 2020 | 
|---|
| Income Tax Disclosure [Abstract] |  | 
| Income Tax Expense Benefit | The income tax expense (benefit) for the years ended December 31, 2020, 2019 and 2018 consists of the following (in millions): 
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
|  | 2020 |  | 2019 |  | 2018 |  
| Current: |  |  |  |  |  |  
| U.S. Federal | $ | 839 |  |  | $ | 570 |  |  | $ | 89 |  |  
| U.S. State and Local | 100 |  |  | 183 |  |  | 7 |  |  
| Non-U.S. | 420 |  |  | 359 |  |  | 374 |  |  
| Total Current | 1,359 |  |  | 1,112 |  |  | 470 |  |  
| Deferred: |  |  |  |  |  |  
| U.S. Federal | (725) |  |  | 255 |  |  | 668 |  |  
| U.S. State and Local | (159) |  |  | (93) |  |  | 75 |  |  
| Non-U.S. | 26 |  |  | (62) |  |  | 15 |  |  
| Total Deferred | (858) |  |  | 100 |  |  | 758 |  |  
| Total Income Tax Expense | $ | 501 |  |  | $ | 1,212 |  |  | $ | 1,228 |  |  | 
| Income Before Income Taxes | Income before income taxes includes the following components (in millions): 
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
|  | 2020 |  | 2019 |  | 2018 |  
| United States | $ | (39) |  |  | $ | 3,972 |  |  | $ | 4,307 |  |  
| Non-U.S. | 1,883 |  |  | 1,680 |  |  | 1,712 |  |  
| Total Income Before Income Taxes: | $ | 1,844 |  |  | $ | 5,652 |  |  | $ | 6,019 |  |  | 
| Reconciliation of Statutory Federal Income Tax Rate to Effective Income Tax Rate | A reconciliation of the statutory federal income tax rate to the effective income tax rate for the years ended December 31, 2020, 2019 and 2018 consists of the following: 
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
|  | 2020 |  | 2019 |  | 2018 |  
| Statutory U.S. federal income tax rate | 21.0 | % |  | 21.0 | % |  | 21.0 | % |  
| 
U.S. state and local income taxes (net of federal benefit) (1)
 | (2.6) |  |  | 1.4 |  |  | 1.4 |  |  
| Non-U.S. tax rate differential | 1.6 |  |  | 0.3 |  |  | 0.2 |  |  
| U.S. federal tax credits | (3.6) |  |  | (1.4) |  |  | (1.1) |  |  
| Goodwill and other asset impairments | 5.1 |  |  | — |  |  | — |  |  
| Net uncertain tax positions | 3.6 |  |  | 0.1 |  |  | (0.6) |  |  
| Non-U.S. valuation allowance release | — |  |  | (1.2) |  |  | — |  |  
| Other | 2.1 |  |  | 1.2 |  |  | (0.5) |  |  
| Effective income tax rate | 27.2 | % |  | 21.4 | % |  | 20.4 | % |  | 
| Deferred Tax Liabilities and Assets | Deferred income tax assets and liabilities are comprised of the following as of December 31, 2020 and 2019 (in millions): 
|  |  |  |  |  |  |  |  |  |  |  |  |  
|  | 2020 |  | 2019 |  
| Fixed assets and capitalized software | $ | (5,355) |  |  | $ | (4,720) |  |  
|  |  |  |  |  
| Operating lease right-of-use assets | (730) |  |  | (685) |  |  
| Other | (501) |  |  | (538) |  |  
| Deferred tax liabilities | (6,586) |  |  | (5,943) |  |  
|  |  |  |  |  
| Pension and postretirement benefits | 3,994 |  |  | 2,522 |  |  
| Loss and credit carryforwards | 325 |  |  | 328 |  |  
| Insurance reserves | 535 |  |  | 413 |  |  
|  |  |  |  |  
| Stock compensation | 183 |  |  | 249 |  |  
| Accrued employee compensation | 583 |  |  | 287 |  |  
| Operating lease liabilities | 736 |  |  | 691 |  |  
| Other | 357 |  |  | 205 |  |  
| Deferred tax assets | 6,713 |  |  | 4,695 |  |  
| Deferred tax assets valuation allowance | (88) |  |  | (54) |  |  
| Deferred tax asset (net of valuation allowance) | 6,625 |  |  | 4,641 |  |  
|  |  |  |  |  
| Net deferred tax asset (liability) | $ | 39 |  |  | $ | (1,302) |  |  
|  |  |  |  |  
| Amounts recognized in the consolidated balance sheets: |  |  |  |  
|  |  |  |  |  
|  |  |  |  |  
| Deferred tax assets | $ | 527 |  |  | $ | 330 |  |  
| Deferred tax liabilities | (488) |  |  | (1,632) |  |  
| Net deferred tax asset (liability) | $ | 39 |  |  | $ | (1,302) |  |  | 
| U.S. State and Local Operating Loss and Credit Carryforwards | Further, we have U.S. state and local operating loss and credit carryforwards as follows (in millions): 
|  |  |  |  |  |  |  |  |  |  |  |  |  
|  | 2020 |  | 2019 |  
| U.S. state and local operating loss carryforwards | $ | 1,253 |  |  | $ | 1,374 |  |  
| U.S. state and local credit carryforwards | $ | 108 |  |  | $ | 110 |  |  | 
| Summarized Activity Related to Unrecognized Tax Benefits | The following table summarizes the activity related to our uncertain tax positions (in millions): 
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
|  | Tax |  | Interest |  | Penalties |  
| Balance at January 1, 2018 | $ | 160 |  |  | $ | 43 |  |  | $ | 9 |  |  
| Additions for tax positions of the current year | 47 |  |  | — |  |  | 1 |  |  
| Additions for tax positions of prior years | 7 |  |  | 10 |  |  | — |  |  
| Reductions for tax positions of prior years for: |  |  |  |  |  |  
| Changes based on facts and circumstances | (43) |  |  | (8) |  |  | (5) |  |  
| Settlements during the period | (1) |  |  | (1) |  |  | — |  |  
| Lapses of applicable statute of limitations | (3) |  |  | — |  |  | — |  |  
| Balance as of December 31, 2018 | 167 |  |  | 44 |  |  | 5 |  |  
| Additions for tax positions of the current year | 6 |  |  | — |  |  | — |  |  
| Additions for tax positions of prior years | 51 |  |  | 13 |  |  | — |  |  
| Reductions for tax positions of prior years for: |  |  |  |  |  |  
| Changes based on facts and circumstances | (45) |  |  | (4) |  |  | (1) |  |  
| Settlements during the period | (3) |  |  | (1) |  |  | — |  |  
| Lapses of applicable statute of limitations | (4) |  |  | — |  |  | — |  |  
| Balance as of December 31, 2019 | 172 |  |  | 52 |  |  | 4 |  |  
| Additions for tax positions of the current year | 61 |  |  | — |  |  | — |  |  
| Additions for tax positions of prior years | 154 |  |  | 34 |  |  | 2 |  |  
| Reductions for tax positions of prior years for: |  |  |  |  |  |  
| Changes based on facts and circumstances | (54) |  |  | (24) |  |  | (2) |  |  
| Settlements during the period | — |  |  | (1) |  |  | — |  |  
| Lapses of applicable statute of limitations | — |  |  | — |  |  | — |  |  
| Balance as of December 31, 2020 | $ | 333 |  |  | $ | 61 |  |  | $ | 4 |  |  |