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, 2013	$ 559,965	$ 1,257	$ 291	$ 296,932	$ (24)	$ 261,509
Balance, Shares at Dec. 31, 2013		12,568	2,906
Net earnings (loss)	32,688					32,688
Unrealized gain (loss) on short-term investments	(173)				(173)
Changes in pension plan assets and benefit obligations	10				10
Amortization of unearned stock-based compensation	4,753			4,753
Issuance of non-voting common stock (upon vesting of restricted stock units)	1	$ 1
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		12
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(178)			(178)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, shares		(4)
Other	2			26	(24)
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 gain (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 gain (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)
Cancellation of restricted non-voting stock units (performance not met)	$ (4,929)			$ (4,929)
Cancellation of restricted non-voting stock units (performance not met), shares	0	0	0	0	0	0
Retirement of treasury stock, 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