Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Revision Of Prior Period Accounting Standards Update Adjustment [Member]	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Earnings (Deficit) [Member]	Accumulated Earnings (Deficit) [Member] Revision Of Prior Period Accounting Standards Update Adjustment [Member]
BALANCE at Dec. 31, 2018	$ 355,203	$ 1,035	$ 852	$ (220,700)	$ 628,295	$ (298)	$ (52,946)	$ 1,035
BALANCE, shares at Dec. 31, 2018			85,174,000
BALANCE, shares at Dec. 31, 2018				(15,401,000)
NET INCOME	69,982						69,982
Foreign currency translation	(41)					(41)
Unrealized gain (loss) on investments	683					683
COMPREHENSIVE INCOME	70,624
Treasury stock purchased	(3,875)			$ (3,875)
Treasury stock purchased, shares				(235,000)
Share-based compensation expense:
Stock option plans	1,690				1,690
Restricted stock award plans	7,569				7,569
Employee stock purchase plan	15				15
Restricted stock award plans	(7,569)				(7,569)
Common stock issued under:
Stock option plans	$ 1,489		$ 3		1,486
Stock option plan, shares	258,621		259,000
Restricted stock award plans	$ (2,739)		$ 5	$ (2,740)	(4)
Restricted stock award plan, shares			504,000	(166,000)
Employee stock purchase plan	284				284
Employee stock purchase plan, shares			16,000
BALANCE at Dec. 31, 2019	431,295		$ 860	$ (227,315)	639,335	344	18,071
BALANCE, shares at Dec. 31, 2019			85,953,000
BALANCE, shares at Dec. 31, 2019				(15,802,000)
NET INCOME	124,264						124,264
Foreign currency translation	199					199
Unrealized gain (loss) on investments	(179)					(179)
COMPREHENSIVE INCOME	124,284
Treasury stock purchased	(17,862)			$ (17,862)
Treasury stock purchased, shares				(1,320,000)
Share-based compensation expense:
Stock option plans	1,134				1,134
Restricted stock award plans	12,227				12,227
Employee stock purchase plan	18				18
Restricted stock award plans	(12,227)				(12,227)
Common stock issued under:
Stock option plans	$ 5,384		$ 10		5,374
Stock option plan, shares	1,040,304		1,040,000
Restricted stock award plans	$ (912)		$ 2	$ (911)	(3)
Restricted stock award plan, shares			243,000	(81,000)
Employee stock purchase plan	339		$ 1		338
Employee stock purchase plan, shares			29,000
BALANCE at Dec. 31, 2020	$ 555,907		$ 873	$ (246,088)	658,423	364	142,335
BALANCE, shares at Dec. 31, 2020	87,264,910		87,265,000
BALANCE, shares at Dec. 31, 2020	(17,202,546)			(17,203,000)
NET INCOME	$ 109,637						109,637
Foreign currency translation	(177)					(177)
Unrealized gain (loss) on investments	(283)					(283)
COMPREHENSIVE INCOME	109,177
Treasury stock purchased	(25,296)			$ (25,296)
Treasury stock purchased, shares				(2,313,000)
Share-based compensation expense:
Stock option plans	464				464
Restricted stock award plans	(14,495)				(14,495)
Employee stock purchase plan	13				13
Restricted stock award plans	14,495				14,495
Common stock issued under:
Stock option plans	$ 549		$ 1		548
Stock option plan, shares	103,407		103,000
Restricted stock award plans	$ (5,511)		$ 13	$ (5,511)	(13)
Restricted stock award plan, shares			1,329,000	(460,000)
Employee stock purchase plan	312				312
Employee stock purchase plan, shares			27,000
BALANCE at Dec. 31, 2021	$ 650,110		$ 887	$ (276,895)	$ 674,242	$ (96)	$ 251,972
BALANCE, shares at Dec. 31, 2021	88,724,438		88,724,000
BALANCE, shares at Dec. 31, 2021	(19,975,776)			(19,976,000)