Condensed Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Non-Voting Common Stock [Member]	Voting Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2016	$ 599,778	$ 1,278	$ 291	$ 304,246	$ (478)	$ 294,441
Balance, Shares at Dec. 31, 2016		12,779	2,906
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(21,798)					(21,798)
Unrealized gain on short-term investments	235				235
Changes in pension plan assets and benefit obligations	(1)				(1)
Amortization of unearned stock-based compensation	3,074			3,074
Issuance of non-voting common stock (upon vesting of restricted stock units)	3	$ 13		(10)
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		136
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(258)	$ (2)		(256)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, Shares		(22)
Retirement of treasury stock	1,028					1,028
Balance at Sep. 30, 2017	582,061	$ 1,289	$ 291	307,054	(244)	273,671
Balance, Shares at Sep. 30, 2017		12,893	2,906
Balance at Dec. 31, 2017	612,654	$ 1,289	$ 291	308,353	(280)	303,001
Balance, Shares at Dec. 31, 2017		12,897	2,906
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(35,607)					(35,607)
Unrealized gain on short-term investments	278				278
Changes in pension plan assets and benefit obligations	(1)				(1)
Amortization of unearned stock-based compensation	4,250			4,250
Currency translation adjustment	(4,091)				(4,091)
Issuance of non-voting common stock (upon vesting of restricted stock units)	3	$ 3		0
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		27
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(101)	$ (1)		(100)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, Shares		(10)
Balance at Sep. 30, 2018	$ 577,385	$ 1,291	$ 291	$ 312,503	$ (4,094)	$ 267,394
Balance, Shares at Sep. 30, 2018		12,914	2,906