COMMITMENTS AND CONTINGENCIES (Details)	12 Months Ended		444 Months Ended
	Dec. 31, 2016 USD ($) site union	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) site union	Dec. 31, 2012 USD ($) site
Loss Contingencies [Line Items]
Letter of credit provided	$ 210,000,000		$ 210,000,000
Gas supply related demand charges and reservation fees	5,600,000
Amount accrued related to all claims	$ 600,000	$ 800,000	$ 600,000
Percentage of workforce represented by unions	62.00%		62.00%
Number of unions | union	2		2
Number of sites for environmental cleanup | site	12		12
Number of sites covered by insurance | site				11
Insurance policy limit				$ 50,000,000.0
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning of Year	$ 123,194,000	124,308,000
Accruals	61,933,000	18,747,000	$ 420,200,000
Expenditures	(32,080,000)	(19,861,000)	(267,200,000)
End of Year	153,047,000	$ 123,194,000	153,047,000
Pending litigation | Pricing dispute related to a long-term gas supply contract
Loss Contingencies [Line Items]
Counter-suit amount sought	14,900,000
Standby Letter of Credit
Loss Contingencies [Line Items]
Letter of credit provided	$ 25,200,000		$ 25,200,000
Environmental Restoration Costs | Sites comprising the majority of the environmental remediation
Accrual for Environmental Loss Contingencies [Roll Forward]
Number of sites affected by environmental remediation | site	6		6
Environmental Restoration Costs | Minimum
Accrual for Environmental Loss Contingencies [Roll Forward]
Estimated future costs to clean up sites	$ 153,000,000		$ 153,000,000
Environmental Restoration Costs | Minimum | Sites comprising the majority of the environmental remediation
Accrual for Environmental Loss Contingencies [Roll Forward]
Estimated future costs to clean up sites	141,200,000		141,200,000
Environmental Restoration Costs | Maximum
Accrual for Environmental Loss Contingencies [Roll Forward]
Estimated future costs to clean up sites	257,100,000		257,100,000
Environmental Restoration Costs | Maximum | Sites comprising the majority of the environmental remediation
Accrual for Environmental Loss Contingencies [Roll Forward]
Estimated future costs to clean up sites	$ 246,700,000		$ 246,700,000