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, 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)
Issuance of common stock under employee stock purchase plan (in shares)	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
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)
Issuance of common stock under employee stock purchase plan (in shares)	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
Stock options exercised and stock awards (in shares)	1,219,000							1,205,281
Stock options exercised and stock awards			12	22,355				$ 22,367
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan				15,606				15,606
Vesting of restricted share awards (in shares)	7,000	199,000
Vesting of restricted share awards			3	(1,518)				(1,515)
Issuance of common stock under employee stock purchase plan (in shares)	113,000
Issuance of common stock under employee stock purchase plan			1	2,474				2,475
Common stock repurchases (in shares)		(975,000)
Common stock repurchases					(33,800)			(33,800)
Other comprehensive income							286	286
Net income						172,129		172,129
Balance (in shares) at Dec. 31, 2017	31,345,000	8,469,000
Balance at Dec. 31, 2017			$ 454	$ 348,044	$ (120,682)	$ 812,557		$ 1,040,373