Consolidated Statements of Shareholders' Equity (USD $) In Thousands	Total	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Voting Common Stock	Non-Voting Common Stock
Balance at Dec. 31, 2009	$ 465,448	$ 291,403	$ (13)	$ 172,527	$ 285	$ 1,246
Balance, Shares at Dec. 31, 2009					2,853	12,459
Net earnings	7,117			7,117
Unrealized gain 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	291,403	(162)	179,644	285	1,246
Balance, Shares at Dec. 31, 2010					2,853	12,459
Net earnings	4,852			4,852
Unrealized gain 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	291,403	(93)	184,496	285	1,246
Balance, Shares at Dec. 31, 2011					2,853	12,459
Net earnings	18,057			18,057
Unrealized gain on short-term investments	55		55
Changes in pension plan assets and benefit obligations	(13)		(13)
Total comprehensive income	18,099
Issuance of non-voting common stock ( upon vesting of restricted stock units), Shares						106
Issuance of non-voting common stock ( upon vesting of restricted stock units)	4,179	4,179
Balance at Dec. 31, 2012	$ 499,615	$ 295,582	$ (51)	$ 202,553	$ 285	$ 1,246
Balance, Shares at Dec. 31, 2012					2,853	12,565