Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement Of Cash Flows [Abstract]
Net cash flows used in operating activities	$ (10,852)	$ (5,489)
Cash flows from investing activities:
Purchases of property, plant and equipment	(2,949)	(3,126)
Purchases of long-term marketable securities	(89)	(20)
Proceeds from sale of long-term marketable securities	128	85
Net cash flows used in investing activities	(2,910)	(3,061)
Cash flows from financing activities:
Dividends paid	0	(1,104)
Repayment of debt	(178)	(932)
Proceeds from Revolving Credit and Security Agreement (Note 7)	16,052	0
Proceeds from credit facility	0	8,795
Payments on credit facility	0	(15,941)
Net cash flows provided by (used in) financing activities	15,874	(9,182)
Effect of exchange rate changes on cash and cash equivalents	142	174
Net increase (decrease) in cash and cash equivalents	2,254	(17,558)
Cash and cash equivalents at beginning of period	20,700	38,579
Cash and cash equivalents at end of period	22,954	21,021
Supplemental information:
Income tax payments	82	202
Interest payments	240	721
Non-cash investing activities:
Purchases of property, plant and equipment included in accounts payable	$ 737	$ 344