CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (USD $) In Thousands	Total	Voting Common Stock [Member]	Non-Voting Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2011	$ 477,337	$ 285	$ 1,246	$ 291,403	$ (93)	$ 184,496
Balance, Shares at Dec. 31, 2011		2,853	12,459
Net earnings	8,232					8,232
Unrealized gain on short-term Investments	67				67
Changes in pension plan assets and benefit obligations	(12)				(12)
Balance at Jun. 30, 2012	485,624	285	1,246	291,403	(38)	192,728
Balance, Shares at Jun. 30, 2012		2,853	12,459
Balance at Dec. 31, 2012	499,615	285	1,246	295,582	(51)	202,553
Balance, Shares at Dec. 31, 2012		2,853	12,565
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	$ 291	$ 1,257	$ 295,528	$ (80)	$ 230,907
Balance, Shares at Jun. 30, 2013		2,906	12,568