FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Values and Carrying Values of Long-Term Debt and Line of Credit (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
FAIR VALUE OF FINANCIAL INSTRUMENTS
Long-term debt (including current maturities) and line of credit, carrying value	$ 5,475	$ 6,377
Long-term debt (including current maturities) and line of credit, fair value (Level 2)	6,943	7,441
Fixed interest rate
FAIR VALUE OF FINANCIAL INSTRUMENTS
Long-term debt (including current maturities) and line of credit, carrying value	5,250	5,922
Long-term debt (including current maturities) and line of credit, fair value (Level 2)	6,718	6,986
Variable interest rate
FAIR VALUE OF FINANCIAL INSTRUMENTS
Long-term debt (including current maturities) and line of credit, carrying value	225	455
Long-term debt (including current maturities) and line of credit, fair value (Level 2)	$ 225	$ 455