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]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss) [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	8,047						8,047
Unrealized gain (loss) on short-term investments	24					24
Changes in pension plan assets and benefit obligation	6					6
Amortization of unearned stock-based compensation	1,729			1,729
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		11
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(176)			(176)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, shares		(4)
Balance at Jun. 30, 2014	569,596	$ 1,258	$ 291	298,845		6	269,556
Balance, Shares at Jun. 30, 2014		12,575	2,906
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	$ (252)	$ (197)	$ 311,014
Balance, Shares at Jun. 30, 2015		12,676	2,906