CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement Of Income and Comprehensive Income [Abstract]
Net income (loss)	$ 57,334	$ (38,942)	$ 104,808	$ (120,139)
Other comprehensive income (loss):
Pension adjustment and reclassification adjustment, net of taxes	135	126	405	380
Foreign currency translation	353	(144)	584	712
Unrealized gain (loss) on securities, net of taxes	9	7	(12)	(2)
Comprehensive income (loss)	$ 57,831	$ (38,953)	$ 105,785	$ (119,049)