Consolidated Statements of Shareholders' Equity (USD $) In Thousands	Total	Voting Common Stock	Non-Voting Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings
Balance at Dec. 31, 2008	$ 452,396	$ 285	$ 1,245	$ 291,262	$ 45	$ 159,559
Balance, shares at Dec. 31, 2008		2,853	12,449
Net earnings	12,968					12,968
Changes in pension plan assets and benefit obligations	(58)				(58)
Total comprehensive income	12,910
Stock options exercised	142		1	141
Stock options exercised, shares			10
Balance at Dec. 31, 2009	465,448	285	1,246	291,403	(13)	172,527
Balance, shares at Dec. 31, 2009		2,853	12,459
Net earnings	7,117					7,117
Unrealized gain (loss) on short-term investments	(142)				(142)
Changes in pension plan assets and benefit obligations	(7)				(7)
Total comprehensive income	6,968
Balance at Dec. 31, 2010	472,416	285	1,246	291,403	(162)	179,644
Balance, shares at Dec. 31, 2010		2,853	12,459
Net earnings	4,852					4,852
Unrealized gain (loss) on short-term investments	81				81
Changes in pension plan assets and benefit obligations	(12)				(12)
Total comprehensive income	4,921
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