Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Non-Voting Common Stock [Member]	Voting Common Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2014	$ 597,068	$ 1,258	$ 291	$ 301,533	$ (211)	$ 294,197
Balance, Shares at Dec. 31, 2014		12,576	2,906
Net earnings (loss)	26,924					26,924
Unrealized loss on short-term investments	(384)				(384)
Changes in pension plan assets and benefit obligations	4				4
Amortization of unearned stock-based compensation	5,799			5,799
Issuance of non-voting common stock (upon vesting of restricted stock units)	18	$ 18
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		179
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(2,203)	$ (7)		(2,196)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, shares		(70)
Purchase and retirement of treasury stock	(252)			(252)
Other	24				24
Balance at Dec. 31, 2015	626,998	$ 1,269	$ 291	304,884	(567)	321,121
Balance, Shares at Dec. 31, 2015		12,685	2,906
Net earnings (loss)	(26,680)					(26,680)
Unrealized loss on short-term investments	113				113
Changes in pension plan assets and benefit obligations	(24)				(24)
Amortization of unearned stock-based compensation	4,819			4,819
Issuance of non-voting common stock (upon vesting of restricted stock units)	12	$ 12
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		130
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(531)	$ (3)		(528)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, shares		(28)
Purchase and retirement of treasury stock	(4,929)			(4,929)
Other	0
Other, shares		(8)
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
Net earnings (loss)	7,532					7,532
Unrealized loss on short-term investments	200				200
Changes in pension plan assets and benefit obligations	(2)				(2)
Amortization of unearned stock-based compensation	4,374			4,374
Cumulative effect adjustment of unrecognized tax benefits	1,028					1,028
Issuance of non-voting common stock (upon vesting of restricted stock units)	0	$ 11		(11)
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		140
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(256)			(256)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, shares		(22)
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