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FAIR VALUE MEASUREMENTS - FAIR VALUE OF ASSETS AND LIABILITIES (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable securities | $ 1,435 |
Fair Value, Measurements, Recurring [Member] | Fuel hedge contracts [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative instruments, assets | 22 | $ 20
Fair Value, Measurements, Recurring [Member] | 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
Fair Value, Measurements, Recurring [Member] | Fuel hedge contracts [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative instruments, assets | 22 | 20
Fair Value, Measurements, Recurring [Member] | Interest rate swaps agreements [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative instruments, assets | 9 |
Derivative instruments, liabilities | (8) | (5)
Fair Value, Measurements, Recurring [Member] | 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 |
Derivative instruments, liabilities | 0 | 0
Fair Value, Measurements, Recurring [Member] | Interest rate swaps agreements [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative instruments, assets | 9 |
Derivative instruments, liabilities | (8) | (5)
Fair Value, Measurements, Recurring [Member] | U.S. government and agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 328 | 287
Fair Value, Measurements, Recurring [Member] | U.S. government and agency securities | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 328 | 287
Fair Value, Measurements, Recurring [Member] | U.S. government and agency securities | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 0 | 0
Fair Value, Measurements, Recurring [Member] | Foreign government bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 43 | 36
Fair Value, Measurements, Recurring [Member] | Foreign government bonds | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 0 | 0
Fair Value, Measurements, Recurring [Member] | Foreign government bonds | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 43 | 36
Fair Value, Measurements, Recurring [Member] | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 209 | 138
Fair Value, Measurements, Recurring [Member] | Asset-backed securities | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 0 | 0
Fair Value, Measurements, Recurring [Member] | Asset-backed securities | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 209 | 138
Fair Value, Measurements, Recurring [Member] | Mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 99 | 89
Fair Value, Measurements, Recurring [Member] | Mortgage-backed securities | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 0 | 0
Fair Value, Measurements, Recurring [Member] | Mortgage-backed securities | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 99 | 89
Fair Value, Measurements, Recurring [Member] | Corporate notes and bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 726 | 691
Fair Value, Measurements, Recurring [Member] | Corporate notes and bonds | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 0 | 0
Fair Value, Measurements, Recurring [Member] | Corporate notes and bonds | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 726 | 691
Fair Value, Measurements, Recurring [Member] | Municipal securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 22 | 11
Fair Value, Measurements, Recurring [Member] | Municipal securities | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 0 | 0
Fair Value, Measurements, Recurring [Member] | Municipal securities | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | $ 22 | $ 11

X
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