FAIR VALUE MEASUREMENTS - FAIR VALUE OF ASSETS AND LIABILITIES (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 1,145
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	4	$ 22
Fair Value, Measurements, Recurring [Member] | Fuel hedge contracts [Member] | Fair Value, Inputs, 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	4	22
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	10	9
Derivative instruments, liabilities	(7)	(8)
Fair Value, Measurements, Recurring [Member] | Interest rate swaps agreements [Member] | Fair Value, Inputs, 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
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	10	9
Derivative instruments, liabilities	(7)	(8)
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	293	328
Fair Value, Measurements, Recurring [Member] | U.S. government and agency securities | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	293	328
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	26	43
Fair Value, Measurements, Recurring [Member] | Foreign government bonds | Fair Value, Inputs, 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	26	43
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	190	209
Fair Value, Measurements, Recurring [Member] | Asset-backed securities | Fair Value, Inputs, 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	190	209
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	92	99
Fair Value, Measurements, Recurring [Member] | Mortgage-backed securities | Fair Value, Inputs, 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	92	99
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	520	726
Fair Value, Measurements, Recurring [Member] | Corporate notes and bonds | Fair Value, Inputs, 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	520	726
Fair Value, Measurements, Recurring [Member] | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	10	22
Fair Value, Measurements, Recurring [Member] | Municipal securities | Fair Value, Inputs, 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	$ 10	$ 22