Condensed Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Non-Voting Common Stock [Member]	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, 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)		$ 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
Balance at Mar. 31, 2018	597,735	$ 1,290	$ 291	309,672	464	286,018
Balance, Shares at Mar. 31, 2018		12,905	2,906
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(7,092)					(7,092)
Unrealized gain on short-term investments	(4)				(4)
Changes in pension plan assets and benefit obligations	4				4
Amortization of unearned stock-based compensation	813			813
Currency translation adjustment	160				160
Issuance of non-voting common stock (upon vesting of restricted stock units)	(1)	$ 1		(2)
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		(2)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(50)			(50)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, Shares		3
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
Balance at Dec. 31, 2018	473,433	$ 1,291	$ 291	314,316	(3,952)	161,487
Balance, Shares at Dec. 31, 2018		12,919	2,906
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(67,318)					(67,318)
Amortization of unearned stock-based compensation	2,640			2,640
Cash payment of Performance Based stock units	(813)			(813)
Currency translation adjustment	(88)				(88)
Balance at Jun. 30, 2019	407,854	$ 1,291	$ 291	316,143	(4,040)	94,169
Balance, Shares at Jun. 30, 2019		12,919	2,906
Balance at Mar. 31, 2019	439,508	$ 1,291	$ 291	314,818	(3,138)	126,246
Balance, Shares at Mar. 31, 2019		12,919	2,906
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(32,077)					(32,077)
Amortization of unearned stock-based compensation	1,325			1,325
Currency translation adjustment	(902)				(902)
Balance at Jun. 30, 2019	$ 407,854	$ 1,291	$ 291	$ 316,143	$ (4,040)	$ 94,169
Balance, Shares at Jun. 30, 2019		12,919	2,906