CLOSURE AND POST-CLOSURE LIABILITIES (Anticipated Payments) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Environmental Property Sale, Disposal or Abandonment Costs
2021	$ 14,941
2022	7,816
2023	16,181
2024	6,868
2025	5,628
Thereafter	269,477
Undiscounted closure and post-closure liabilities	320,911
Less: Discount at credit-adjusted risk-free rate	(159,164)
Less: Undiscounted estimated closure and post-closure liabilities relating to airspace not yet consumed	(73,821)
Present value of closure and post-closure liabilities	$ 87,926	$ 75,651	$ 69,931