CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Unaudited) (USD $) In Thousands	Total	Non-Voting Common Stock [Member]	Voting Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2012	$ 499,615	$ 1,246	$ 285	$ 295,582	$ (51)	$ 202,553
Balance, Shares at Dec. 31, 2012		12,565	2,853
Net earnings	28,354					28,354
Unrealized gain on short-term investments	(23)				(23)
Changes in pension plan assets and benefit obligations	(6)				(6)
Amortization of unearned stock-based compensation	999			999
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		4
Cancellation of restricted non-voting stock units for tax withholdings on vested shares	(65)	(6)		(59)
Cancellation of restricted non-voting stock units for tax withholdings on vested shares, shares		(1)
Issuance of voting common stock (upon vesting of restricted stock units)	9		9
Issuance of voting common stock (upon vesting of restricted stock units), shares			84
Cancellation of restricted voting stock units for tax withholdings on vested shares	(997)		(3)	(994)
Cancellation of restricted voting stock units for tax withholdings on vested shares, shares			(31)
Balance at Jun. 30, 2013	527,903	1,257	291	295,528	(80)	230,907
Balance, Shares at Jun. 30, 2013		12,568	2,906
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 on short-term investments	24				24
Changes in pension plan assets and benefit obligations	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	$ 298,485	$ 6	$ 269,556
Balance, Shares at Jun. 30, 2014		12,575	2,906