CLOSURE AND POST-CLOSURE LIABILITIES (Anticipated Payments) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Environmental Property Sale, Disposal or Abandonment Costs
2018	$ 6,313
2019	8,957
2020	12,152
2021	8,652
2022	353
Thereafter	285,404
Undiscounted closure and post-closure liabilities	321,831
Less: Discount at credit-adjusted risk-free rate	(161,767)
Less: Undiscounted estimated closure and post-closure liabilities relating to airspace not yet consumed	(99,027)
Present value of closure and post-closure liabilities	$ 61,037	$ 58,331	$ 56,249