CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME [Abstract]
NET INCOME	$ 30,260	$ 28,801	$ 21,361	$ 15,400	$ 8,712	$ 19,677	$ 11,777	$ 5,126	$ 95,822	$ 45,292	$ 107,936
OTHER COMPREHENSIVE LOSS, NET OF TAX:
Defined benefit pension plan adjustments									(2,623)	2,026	(8,155)
Foreign currency translation adjustment									(261)	(3,214)	(638)
Unrealized (loss) gain in fair value of investments									(340)	766	205
Unrealized (loss) gain in fair value of interest rate swap									(24)	(125)	349
Total other comprehensive loss, net of tax									(3,248)	(547)	(8,239)
TOTAL COMPREHENSIVE INCOME									$ 92,574	$ 44,745	$ 99,697