COMMITMENTS AND CONTINGENCIES (Details) (USD $)	12 Months Ended		420 Months Ended
	Dec. 31, 2014 unions sites	Dec. 31, 2013	Dec. 31, 2014 unions sites	Dec. 31, 2012 sites
Loss Contingencies [Line Items]
Letter of credit provided	$ 210,000,000		$ 210,000,000
Monthly gas supply related demand charges and reservation fees	3,900,000
Estimated litigation liability	500,000	500,000	500,000
Percentage of personnel represented in collective bargaining agreements	59.00%		59.00%
Number of unions	2		2
Number of sites for environmental cleanup	12		12
Number of sites covered by insurance				11
Insurance policy limit				50,000,000.0
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning of Year	119,492,000	107,410,000
Accruals	16,453,000	22,264,000	343,900,000
Expenditures	(11,637,000)	(10,182,000)	(219,600,000)
End of Year	124,308,000	119,492,000	124,308,000
Standby Letter of Credit [Member]
Loss Contingencies [Line Items]
Letter of credit provided	25,200,000		25,200,000
Environmental Restoration Costs [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Range of possible loss, minimum	124,300,000		124,300,000
Range of possible loss, maximum	223,300,000		223,300,000
Environmental Restoration Costs [Member] | Sites Comprising the Majority the Environmental Remediation [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Range of possible loss, minimum	110,200,000		110,200,000
Range of possible loss, maximum	$ 198,400,000		$ 198,400,000
Number of sites affected by environmental remediation	6		6