Condensed Consolidated Statements of Shareholders' Equity (Unaudited) - 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]	Treasury Stock [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	24,486					24,486
Unrealized gain (loss) on short-term investments	(3)				(3)
Changes in pension plan assets and benefit obligations	2				2
Amortization of unearned stock-based compensation	5,059			5,059
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		177
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(2,207)	$ (7)		(2,200)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, shares		(69)
Purchase of treasury stock	(252)						$ (252)
Other	24				24
Balance at Sep. 30, 2015	624,195	$ 1,269	$ 291	304,392	(188)	318,683	$ (252)
Balance, Shares at Sep. 30, 2015		12,684	2,906
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	(9,622)					(9,622)
Unrealized gain (loss) on short-term investments	294				294
Changes in pension plan assets and benefit obligations	2				2
Amortization of unearned stock-based compensation	4,334			4,334
Issuance of non-voting common stock (upon vesting of restricted stock units)	12	$ 12
Issuance of voting common stock (upon vesting of restricted stock units), shares		128
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)
Retirement of treasury stock, Shares		(8)
Balance at Sep. 30, 2016	$ 621,487	$ 1,278	$ 291	$ 308,690	$ (271)	$ 311,499
Balance, Shares at Sep. 30, 2016		12,777	2,906