FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Values and Carrying Values of Long-Term Debt and Line of Credit (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
FAIR VALUE OF FINANCIAL INSTRUMENTS
Long-term debt (including current maturities) and line of credit, carrying value	$ 5,475	$ 5,475
Long-term debt (including current maturities) and line of credit, fair value (Level 2)	6,638	6,943
Fixed interest rate
FAIR VALUE OF FINANCIAL INSTRUMENTS
Long-term debt (including current maturities) and line of credit, carrying value	5,250	5,250
Long-term debt (including current maturities) and line of credit, fair value (Level 2)	6,413	6,718
Variable interest rate
FAIR VALUE OF FINANCIAL INSTRUMENTS
Long-term debt (including current maturities) and line of credit, carrying value	225	225
Long-term debt (including current maturities) and line of credit, fair value (Level 2)	$ 225	$ 225