COMMITMENTS AND CONTINGENCIES (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($) sites unions	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Letter of credit provided	$ 210,000
Number of sites for environmental cleanup | sites	12
Monthly gas supply related demand charges and reservation fees	$ 5,900
Estimated litigation liability	$ 600	$ 800
Percentage of personnel represented in collective bargaining agreements	60.00%
Number of unions | unions	2
Standby Letter of Credit
Loss Contingencies [Line Items]
Letter of credit provided	$ 25,200
Revolving Credit Facility
Loss Contingencies [Line Items]
Letters of credit outstanding, amount	800
Letter of credit provided	$ 200,000