Consolidated Statements Of Shareholders’ Equity (USD $) In Thousands, except Share data	Common Class A [Member]	Common Class B [Member]	Common Stock [Member] USD ($)	Additional Paid-in Capital [Member] USD ($)	Treasury Stock [Member] USD ($)	Retained Earnings [Member] USD ($)	Accumulated Other Comprehensive Income (Loss) [Member] USD ($)	Total USD ($)
Balance, amount at Dec. 31, 2009			$ 388	$ 188,116	$ (17,948)	$ 255,669		$ 426,225
Balance, shares (in Shares) at Dec. 31, 2009	26,438,000	10,689,000
Net income						31,282		31,282
Stock options exercised, amount			2	2,486				2,488
Stock options exercised, shares (in Shares)	212,000	11,000
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan				4,468				4,468
Vesting of restricted share awards, amount			1	(1)
Vesting of restricted share awards, shares (in Shares)	83,000
Issuance of common stock under employee stock purchase plan, amount				678				678
Issuance of common stock under employee stock purchase plan, shares (in Shares)	66,000
Other comprehensive income (loss)							(222)	(222)
Balance, amount at Dec. 31, 2010			391	195,747	(17,948)	286,951	(222)	464,919
Balance, shares (in Shares) at Dec. 31, 2010	26,799,000	10,700,000
Net income						55,213		55,213
Stock options exercised, amount			5	6,303				6,308
Stock options exercised, shares (in Shares)	455,000	77,000
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan				5,683				5,683
Vesting of restricted share awards, amount			1	(1)
Vesting of restricted share awards, shares (in Shares)	90,000
Issuance of common stock under employee stock purchase plan, amount			1	837				838
Issuance of common stock under employee stock purchase plan, shares (in Shares)	62,000							62,405
Other comprehensive income (loss)							(1,727)	(1,727)
Balance, amount at Dec. 31, 2011			398	208,569	(17,948)	342,164	(1,949)	531,234
Balance, shares (in Shares) at Dec. 31, 2011	27,406,000	10,777,000
Net income						62,455		62,455
Stock options exercised, amount			4	5,722				5,726
Stock options exercised, shares (in Shares)	439,000	15,000						426,976
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan				7,332				7,332
Vesting of restricted share awards, amount			1	(62)				(61)
Vesting of restricted share awards, shares (in Shares)	83,000
Issuance of common stock under employee stock purchase plan, amount			1	1,066				1,067
Issuance of common stock under employee stock purchase plan, shares (in Shares)	71,000							70,998
Other comprehensive income (loss)							193	193
Balance, amount at Dec. 31, 2012			$ 404	$ 222,627	$ (17,948)	$ 404,619	$ (1,756)	$ 607,946
Balance, shares (in Shares) at Dec. 31, 2012	27,999,000	10,792,000