CLOSURE AND POST-CLOSURE LIABILITIES (Anticipated Payments) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Environmental Property Sale, Disposal or Abandonment Costs
2020	$ 7,571
2021	10,307
2022	14,407
2023	11,329
2024	3,344
Thereafter	277,001
Undiscounted closure and post-closure liabilities	323,959
Less: Discount at credit-adjusted risk-free rate	(158,186)
Less: Undiscounted estimated closure and post-closure liabilities relating to airspace not yet consumed	(90,122)
Present value of closure and post-closure liabilities	$ 75,651	$ 69,931	$ 61,037