CLOSURE AND POST-CLOSURE LIABILITIES (Anticipated Payments) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Environmental Property Sale, Disposal or Abandonment Costs
2019	$ 10,574
2020	9,424
2021	10,941
2022	7,626
2023	9,264
Thereafter	271,314
Undiscounted closure and post-closure liabilities	319,143
Less: Discount at credit-adjusted risk-free rate	(155,999)
Less: Undiscounted estimated closure and post-closure liabilities relating to airspace not yet consumed	(93,213)
Present value of closure and post-closure liabilities	$ 69,931	$ 61,037	$ 58,331