Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member] Voting Common Stock [Member]	Common Stock [Member] Non-Voting Common Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2015	$ 626,998	$ 291	$ 1,269	$ 304,884	$ (567)	$ 321,121
Balance, Shares at Dec. 31, 2015		2,906	12,685
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
Currency translation	0
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, shares			(8)
Balance at Dec. 31, 2016	599,778	$ 291	$ 1,278	304,246	(478)	294,441
Balance, Shares at Dec. 31, 2016		2,906	12,779
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
Currency translation	0
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)
Other	0				0
Other, shares			0
Balance at Dec. 31, 2017	612,654	$ 291	$ 1,289	308,353	(280)	303,001
Balance, Shares at Dec. 31, 2017		2,906	12,897
Net earnings (loss)	(141,514)					(141,514)
Unrealized loss on short-term investments	279				279
Changes in pension plan assets and benefit obligations	(1)				(1)
Amortization of unearned stock-based compensation	6,070			6,070
Currency translation	(3,950)				(3,950)
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			33
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(107)			(107)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, shares			(11)
Balance at Dec. 31, 2018	$ 473,433	$ 291	$ 1,291	$ 314,316	$ (3,952)	$ 161,487
Balance, Shares at Dec. 31, 2018		2,906	12,919