CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) [Abstract]
NET INCOME	$ 8,712	$ 19,677	$ 11,777	$ 5,126	$ 27,722	$ 35,730	$ 28,545	$ 15,939	$ 45,292	$ 107,936	$ 87,296
OTHER COMPREHENSIVE (LOSS) INCOME, NET OF TAX:
Defined benefit pension plan adjustments									2,026	(8,155)	10,910
Foreign currency translation adjustment									(3,214)	(638)	(738)
Unrealized (loss) gain in fair value of investments									766	205	(555)
Unrealized gain in fair value of interest rate swap									(125)	349	578
Total other comprehensive (loss) income, net of tax									(547)	(8,239)	10,195
TOTAL COMPREHENSIVE INCOME									$ 44,745	$ 99,697	$ 97,491