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	(16,983)					(16,983)
Unrealized gain on short-term investments	282				282
Changes in pension plan assets and benefit obligations	(5)				(5)
Amortization of unearned stock-based compensation	1,319			1,319
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax, Portion Attributable to Parent	467				467
Issuance of non-voting common stock (upon vesting of restricted stock units)	1	$ 1		0
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		11
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	0
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, Shares		(3)
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
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	(35,241)					(35,241)
Amortization of unearned stock-based compensation	1,315			1,315
Cash payment of Performance Based stock units	(813)			(813)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax, Portion Attributable to Parent	814				814
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