Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Common Stock [Member] Common Class A [Member]	Common Stock [Member] Common Class B [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Total
Balance (in shares) at Dec. 31, 2013	28,911,000	10,305,000
Balance at Dec. 31, 2013			$ 414	$ 243,154	$ (30,821)	$ 453,836	$ (1,202)	$ 665,381
Stock options exercised and stock awards (in shares)	872,000	30,000
Stock options exercised and stock awards			9	16,216				16,225
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan				11,343				11,343
Vesting of restricted share awards (in shares)	42,000	113,000
Vesting of restricted share awards			1	311				312
Stock Issued During Period, Shares, Employee Stock Purchase Plans	64,000
Issuance of common stock under employee stock purchase plan				1,462				1,462
Issuance of common stock related to business acquisition					1,170			1,170
Common stock repurchases (in shares)		(449,000)
Common stock repurchases					(12,253)			(12,253)
Other comprehensive income							742	742
Net income						79,957		79,957
Balance (in shares) at Dec. 31, 2014	29,889,000	9,999,000
Balance at Dec. 31, 2014			424	272,486	(41,904)	533,793	(460)	764,339
Stock options exercised and stock awards (in shares)	182,000
Stock options exercised and stock awards			2	1,933				1,935
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan				12,384				12,384
Vesting of restricted share awards (in shares)	147,000	150,000
Vesting of restricted share awards			3	(572)				(569)
Stock Issued During Period, Shares, Employee Stock Purchase Plans	86,000
Issuance of common stock under employee stock purchase plan			1	2,063				2,064
Common stock repurchases (in shares)		(56,000)
Common stock repurchases					(1,464)			(1,464)
Other comprehensive income							155	155
Net income						66,053		66,053
Balance (in shares) at Dec. 31, 2015	30,304,000	10,093,000
Balance at Dec. 31, 2015			430	288,294	(43,368)	599,846	(305)	$ 844,897
Stock options exercised and stock awards (in shares)	494,000							466,877
Stock options exercised and stock awards			5	6,152				$ 6,157
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan				12,875				12,875
Vesting of restricted share awards (in shares)	6,000	186,000
Vesting of restricted share awards			2	(725)				(723)
Stock Issued During Period, Shares, Employee Stock Purchase Plans	137,000
Issuance of common stock under employee stock purchase plan			1	2,531				2,532
Common stock repurchases (in shares)	(934,000)	(1,034,000)
Common stock repurchases					(43,514)			(43,514)
Other comprehensive income							19	19
Net income						40,582		40,582
Balance (in shares) at Dec. 31, 2016	30,007,000	9,245,000
Balance at Dec. 31, 2016			$ 438	$ 309,127	$ (86,882)	$ 640,428	$ (286)	$ 862,825