POSTRETIREMENT MEDICAL BENEFITS AND PENSION - Components of Net Periodic Benefit Cost (Details) (Postretirement Medical Benefits, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Postretirement Medical Benefits
Components of net periodic benefit cost:
Service cost	$ 534	$ 440
Interest cost	1,625	1,448
Expected return on plan assets	(2,060)	(1,905)
Amortization of deferred items	741	729
Total net periodic benefit cost	$ 840	$ 712