| INCOME TAXES (Tables)
 | 12 Months Ended | 
| Dec. 31, 2018 | 
|---|
| Income Tax Disclosure [Abstract] |  | 
| Income Tax Expense Benefit | 
The income tax expense (benefit) for the years ended December 31, 2018, 2017 and 2016 consists of the following (in millions):
 
|  |  
|  |  |  |  |  |  |  |  |  |  |  |  |  
|   | 2018 |   | 2017 |   | 2016 |  
| Current: |   |   |   |   |   |  
| U.S. Federal | $ | 89 | 
 |   | $ | 671 | 
 |   | $ | 1,338 | 
 |  
| U.S. State and Local | 7 | 
 |   | 49 | 
 |   | 67 | 
 |  
| Non-U.S. | 374 | 
 |   | 288 | 
 |   | 177 | 
 |  
| Total Current | 470 | 
 |   | 1,008 | 
 |   | 1,582 | 
 |  
| Deferred: |   |   |   |   |   |  
| U.S. Federal | 668 | 
 |   | 1,115 | 
 |   | 98 | 
 |  
| U.S. State and Local | 75 | 
 |   | 118 | 
 |   | 30 | 
 |  
| Non-U.S. | 15 | 
 |   | (9 | ) |   | (11 | ) |  
| Total Deferred | 758 | 
 |   | 1,224 | 
 |   | 117 | 
 |  
| Total Income Tax Expense | $ | 1,228 | 
 |   | $ | 2,232 | 
 |   | $ | 1,699 | 
 |  | 
| Income Before Income Taxes | 
Income before income taxes includes the following components (in millions): 
|  |  
|  |  |  |  |  |  |  |  |  |  |  |  |  
|   | 2018 |   | 2017 |   | 2016 |  
| United States | $ | 4,307 | 
 |   | $ | 5,987 | 
 |   | $ | 4,307 | 
 |  
| Non-U.S. | 1,712 | 
 |   | 1,150 | 
 |   | 814 | 
 |  
| Total Income Before Income Taxes: | $ | 6,019 | 
 |   | $ | 7,137 | 
 |   | $ | 5,121 | 
 |  | 
| 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, 2018, 2017 and 2016 consists of the following:
 
|  |  
|  |  |  |  |  |  |  |  |  |  
|   | 2018 |   | 2017 |   | 2016 |  
| Statutory U.S. federal income tax rate | 21.0 |  % |   | 35.0 |  % |   | 35.0 |  % |  
| U.S. state and local income taxes (net of federal benefit) | 1.4 | 
 |   | 1.5 | 
 |   | 1.5 | 
 |  
| Non-U.S. tax rate differential | 0.2 | 
 |   | (2.0 | ) |   | (2.4 | ) |  
| U.S. federal tax credits | (1.1 | ) |   | (1.8 | ) |   | (1.2 | ) |  
| Income tax benefit from the Tax Cuts and Jobs Act and other non-U.S. tax law changes | — | 
 |   | (3.6 | ) |   | — | 
 |  
| 
Defined benefit plans mark-to-market charge tax rate differential (1)
 | — | 
 |   | 1.5 | 
 |   | — | 
 |  
| Other | (1.1 | ) |   | 0.7 | 
 |   | 0.3 | 
 |  
| Effective income tax rate | 20.4 |  % |   | 31.3 |  % |   | 33.2 |  % |  | 
| Deferred Tax Liabilities and Assets | 
Deferred income tax assets and liabilities are comprised of the following at December 31, 2018 and 2017 (in millions):
 
|  |  
|  |  |  |  |  |  |  |  |  
|   | 2018 |   | 2017 |  
| Fixed assets and capitalized software | $ | (4,010 | ) |   | $ | (3,288 | ) |  
| Other | (493 | ) |   | (532 | ) |  
| Deferred tax liabilities | (4,503 | ) |   | (3,820 | ) |  
|   |   |   |   |  
| Pension and postretirement benefits | 1,743 | 
 |   | 1,877 | 
 |  
| Loss and credit carryforwards | 298 | 
 |   | 323 | 
 |  
| Insurance reserves | 437 | 
 |   | 449 | 
 |  
| Stock compensation | 189 | 
 |   | 182 | 
 |  
| Accrued employee compensation | 274 | 
 |   | 266 | 
 |  
| Other | 196 | 
 |   | 359 | 
 |  
| Deferred tax assets | 3,137 | 
 |   | 3,456 | 
 |  
| Deferred tax assets valuation allowance | (112 | ) |   | (126 | ) |  
| Deferred tax asset (net of valuation allowance) | 3,025 | 
 |   | 3,330 | 
 |  
|   |   |   |   |  
| Net deferred tax asset (liability) | $ | (1,478 | ) |   | $ | (490 | ) |  
|   |   |   |   |  
| Amounts recognized in the consolidated balance sheets: |   |   |   |  
| Deferred tax assets | $ | 141 | 
 |   | $ | 266 | 
 |  
| Deferred tax liabilities | (1,619 | ) |   | (756 | ) |  
| Net deferred tax asset (liability) | $ | (1,478 | ) |   | $ | (490 | ) |  | 
| U.S. State and Local Operating Loss and Credit Carryforwards | 
In addition, we have U.S. state and local operating loss and credit carryforwards as follows (in millions): 
|  |  
|  |  |  |  |  |  |  |  |  
|   | 2018 |   | 2017 |  
| U.S. state and local operating loss carryforwards | $ | 1,014 | 
 |   | $ | 1,215 | 
 |  
| U.S. state and local credit carryforwards | $ | 80 | 
 |   | $ | 83 | 
 |  | 
| 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, 2016 | $ | 148 | 
 |   | $ | 53 | 
 |   | $ | 6 | 
 |  
| Additions for tax positions of the current year | 17 | 
 |   | — | 
 |   | — | 
 |  
| Additions for tax positions of prior years | 20 | 
 |   | 10 | 
 |   | — | 
 |  
| Reductions for tax positions of prior years for: |   |   |   |   |   |  
| Changes based on facts and circumstances | (41 | ) |   | (13 | ) |   | — | 
 |  
| Settlements during the period | — | 
 |   | — | 
 |   | — | 
 |  
| Lapses of applicable statute of limitations | — | 
 |   | — | 
 |   | — | 
 |  
| Balance at December 31, 2016 | 144 | 
 |   | 50 | 
 |   | 6 | 
 |  
| Additions for tax positions of the current year | 16 | 
 |   | — | 
 |   | — | 
 |  
| Additions for tax positions of prior years | 33 | 
 |   | 14 | 
 |   | 3 | 
 |  
| Reductions for tax positions of prior years for: |   |   |   |   |   |  
| Changes based on facts and circumstances | (24 | ) |   | (18 | ) |   | — | 
 |  
| Settlements during the period | (6 | ) |   | (3 | ) |   | — | 
 |  
| Lapses of applicable statute of limitations | (3 | ) |   | — | 
 |   | — | 
 |  
| Balance at December 31, 2017 | 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 at December 31, 2018 | $ | 167 | 
 |   | $ | 44 | 
 |   | $ | 5 | 
 |  |