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	(18,521)					(18,521)
Unrealized gain on short-term investments	210				210
Changes in pension plan assets and benefit obligations	14				14
Amortization of unearned stock-based compensation	1,816			1,816
Issuance of non-voting common stock (upon vesting of restricted stock units)	13	$ 13
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		134
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(277)	$ (2)		(275)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, Shares		(21)
Retirement of treasury stock	1,028					1,028
Balance at Jun. 30, 2017	584,061	$ 1,289	$ 291	305,787	(254)	276,948
Balance, Shares at Jun. 30, 2017		12,892	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	(24,075)					(24,075)
Unrealized gain on short-term investments	278				278
Changes in pension plan assets and benefit obligations	(1)				(1)
Amortization of unearned stock-based compensation	2,132			2,132
Currency translation adjustment	627				627
Issuance of non-voting common stock (upon vesting of restricted stock units)	0	$ 2		(2)
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		9
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(50)			(50)
Balance at Jun. 30, 2018	$ 591,565	$ 1,291	$ 291	$ 310,433	$ 624	$ 278,926
Balance, Shares at Jun. 30, 2018		12,906	2,906