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
Statement of Comprehensive Income [Abstract]
Net (loss) earnings	$ (17,058)	$ (4,967)	$ 4,277	$ (8,932)	$ 2,438	$ 7,669	$ 6,454	$ 10,363	$ (26,680)	$ 26,924	$ 32,688
Net unrealized gain (loss) on short-term investments									241	(641)	(285)
Other unrealized gain (loss)										24	(24)
Changes in pension plan assets and benefit obligations									(39)	7	17
Tax effect of the above-listed adjustments									(113)	254	105
Total comprehensive (loss) income									$ (26,591)	$ 26,568	$ 32,501