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FAIR VALUE MEASUREMENTS - FAIR VALUE OF ASSETS AND LIABILITIES (Details) - Recurring [Member] - USD ($) $ in Millions | Sep. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | $ 1,382 | $ 1,131
Assets, Fair Value Disclosure | 1,391 | 1,145
Financial and Nonfinancial Liabilities, Fair Value Disclosure | (14) | (7)
Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 384 | 293
Assets, Fair Value Disclosure | 384 | 293
Financial and Nonfinancial Liabilities, Fair Value Disclosure | 0 | 0
Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 998 | 838
Assets, Fair Value Disclosure | 1,007 | 852
Financial and Nonfinancial Liabilities, Fair Value Disclosure | (14) | (7)
Fuel hedge contracts [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative instruments, assets | 7 | 4
Fuel hedge contracts [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative instruments, assets | 0 | 0
Fuel hedge contracts [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative instruments, assets | 7 | 4
Interest rate swaps agreements [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative instruments, assets | 2 | 10
Derivative instruments, liabilities | 14 | 7
Interest rate swaps agreements [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative instruments, assets | 0 | 0
Derivative instruments, liabilities | 0 | 0
Interest rate swaps agreements [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative instruments, assets | 2 | 10
Derivative instruments, liabilities | 14 | 7
U.S. government and agency securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 379 | 293
U.S. government and agency securities [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 379 | 293
U.S. government and agency securities [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 0 | 0
Equity Funds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 5 | 0
Equity Funds [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 5 | 0
Equity Funds [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 0 | 0
Foreign government bonds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 25 | 26
Foreign government bonds [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 0 | 0
Foreign government bonds [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 25 | 26
Asset-backed securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 201 | 190
Asset-backed securities [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 0 | 0
Asset-backed securities [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 201 | 190
Mortgage-backed securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 142 | 92
Mortgage-backed securities [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 0 | 0
Mortgage-backed securities [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 142 | 92
Corporate notes and bonds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 609 | 520
Corporate notes and bonds [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 0 | 0
Corporate notes and bonds [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 609 | 520
Municipal securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 21 | 10
Municipal securities [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | 0 | 0
Municipal securities [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | $ 21 | $ 10

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