POSTRETIREMENT MEDICAL BENEFITS - Postretirement Medicial Benefit Obligations Balance Sheet Disclosures (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Change in benefit obligation:
Net benefit obligation at end of year	$ 211,100	$ 158,700
Amounts recognized in the balance sheet consist of:
Noncurrent liabilities	(49,779)	(24,176)
Postretirement Medical Benefits [Member]
Change in benefit obligation:
Net benefit obligation at beginning of year	284,329	333,842
Service cost	3,289	4,436	3,555
Interest cost	12,814	12,139	12,363
Plan participant contributions	149	132
Actuarial loss (gain)	19,441	(53,230)
Gross benefits paid	(14,860)	(14,220)
Federal subsidy on benefits paid	1,256	1,230
Net benefit obligation at end of year	306,418	284,329	333,842
Change in plan assets:
Employer contributions	14,711	14,088
Plan participant contributions	149	132
Gross benefits paid	(14,860)	(14,220)
Fair value of plan assets at end of year	0	0
Unfunded status at end of year	(306,418)	(284,329)
Amounts recognized in the balance sheet consist of:
Current liabilities	(13,263)	(13,955)
Noncurrent liabilities	(293,156)	(270,374)
Accumulated other comprehensive loss	39,716	20,292
Net amount recognized at end of year	(266,703)	(264,037)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss	44,800	26,012
Prior service cost	(5,084)	(5,720)
Accumulated other comprehensive loss	$ 39,716	$ 20,292