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	16,817					16,817
Unrealized gain (loss) on short-term investments	(10)				(10)
Amortization of unearned stock-based compensation	3,359			3,359
Issuance of non-voting common stock (upon vesting of restricted stock units)	17	$ 17
Issuance of non-voting common stock (upon vesting of restricted stock units), shares		166
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(2,103)	$ (7)		(2,096)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, shares		(66)
Purchase of treasury stock	(252)						$ (252)
Other	24				24
Balance at Jun. 30, 2015	614,920	$ 1,268	$ 291	302,796	(197)	311,014	$ (252)
Balance, Shares at Jun. 30, 2015		12,676	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	(4,655)					(4,655)
Unrealized gain (loss) on short-term investments	611				611
Changes in pension plan assets and benefit obligations	1				1
Amortization of unearned stock-based compensation	2,882			2,882
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		121
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(503)	$ (3)		(500)
Cancellation of restricted voting stock units for tax withholdings on vested shares, shares		(27)
Retirement of treasury stock, Shares		(8)
Balance at Jun. 30, 2016	$ 625,346	$ 1,278	$ 291	$ 307,266	$ 45	$ 316,466
Balance, Shares at Jun. 30, 2016		12,771	2,906