POSTRETIREMENT MEDICAL BENEFITS - Postretirement Medicial Benefit Obligations Balance Sheet Disclosures (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Change in benefit obligation:
Net benefit obligation at end of year	$ 176,500	$ 105,100
Amounts recognized in the balance sheet consist of:
Noncurrent liabilities	(54,250)	(28,991)
Postretirement Medical Benefits [Member]
Change in benefit obligation:
Net benefit obligation at beginning of year	258,641	210,860
Liability acquired	49,241	0
Service cost	3,555	493	539
Interest cost	12,363	10,510	10,498
Plan participant contributions	129	200
Actuarial (gain) loss	22,342	49,135
Gross benefits paid	(13,544)	(13,604)
Federal subsidy on benefits paid	1,115	1,047
Net benefit obligation at end of year	333,842	258,641	210,860
Change in plan assets:
Employer contributions	13,415	13,404
Plan participant contributions	129	200
Gross benefits paid	(13,544)	(13,604)
Fair value of plan assets at end of year	0	0
Unfunded status at end of year	(333,842)	(258,641)
Amounts recognized in the balance sheet consist of:
Current liabilities	(14,068)	(13,179)
Noncurrent liabilities	(319,775)	(245,462)
Accumulated other comprehensive loss (gain)	77,528	57,758
Net amount recognized at end of year	(256,315)	(200,883)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss (gain)	83,884	64,656
Prior service credit	(6,356)	(6,991)
Transition obligation	0	93
Accumulated other comprehensive loss (gain)	$ 77,528	$ 57,758