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FAIR VALUE MEASUREMENTS (Fair Value of Senior Notes) (Details) - Fair value, inputs, Level 2 [Member] - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Fair Value Measurements    
Carrying amount of senior notes $ 4,124,168 $ 4,123,217
Estimated fair value of senior notes $ 4,402,833 $ 4,647,595