CONSOLIDATED STATEMENTS OF COMPREHENSIVE (LOSS) INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Other Comprehensive Income [Abstract]
NET INCOME (LOSS)	$ 162,511	$ (141,246)	$ (99,561)
OTHER COMPREHENSIVE INCOME (LOSS), NET OF TAXES:
Pension adjustment	1,033	(788)	1,531
Foreign currency translation	391	652	(1,801)
Unrealized (loss) gain on securities, net reclassification adjustments	(24)	3	134
COMPREHENSIVE INCOME (LOSS)	$ 163,911	$ (141,379)	$ (99,697)