FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated fair values and carrying values of our long-term debt and line of credit (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt (including current maturities) and line of credit, carrying value	$ 6,344	$ 5,992
Fair Value, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt (including current maturities) and line of credit, fair value	6,995	7,048
Fixed rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt (including current maturities) and line of credit, carrying value	5,694	5,768
Fixed rate | Fair Value, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt (including current maturities) and line of credit, fair value	6,345	6,823
Variable rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt (including current maturities) and line of credit, carrying value	650	224
Variable rate | Fair Value, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt (including current maturities) and line of credit, fair value	$ 650	$ 225