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, 2017	$ 296,205		$ 843	$ (217,355)	$ 621,008	$ (164)	$ (108,127)
BALANCE, shares at Dec. 31, 2017			84,280,000
BALANCE, shares at Dec. 31, 2017				(15,162,000)
NET INCOME	55,181						55,181
Foreign currency translation	(100)					(100)
Unrealized gain (loss) on investments	(34)					(34)
COMPREHENSIVE INCOME	55,047
Share-based compensation expense:
Stock option plans	1,956				1,956
Restricted stock award plans	3,641				3,641
Employee stock purchase plan	17				17
Restricted stock award plans	(3,641)				(3,641)
Common stock issued under:
Stock option plans	$ 1,363		$ 2		1,361
Stock option plan, shares	161,072		161,000
Restricted stock award plans	$ (3,345)		$ 7	$ (3,345)	(7)
Restricted stock award plan, shares			709,000	(239,000)
Employee stock purchase plan	319				319
Employee stock purchase plan, shares			24,000
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	85,953,253		85,953,000
BALANCE, shares at Dec. 31, 2019	(15,801,896)			(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)