Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Earnings (Deficit) [Member]
BALANCE at Dec. 31, 2016	$ 321,503	$ 835	$ (216,169)	$ 613,325	$ (258)	$ (76,230)
BALANCE, shares at Dec. 31, 2016		83,538,000
BALANCE, shares at Dec. 31, 2016			(15,019,000)
Net income (loss)	(31,897)					(31,897)
Foreign currency translation	288				288
Unrealized gain (loss) on investments	(194)				(194)
COMPREHENSIVE INCOME (LOSS)	(31,803)
Share-based compensation expense:
Stock option plans	1,585			1,585
Restricted stock award plans	3,366			3,366
Employee stock purchase plan	19			19
Common stock issued under:
Stock option plans	$ 2,360	$ 3		2,357
Stock option plan, shares	288,618	289,000
Restricted stock award plans	$ (1,187)	$ 4	$ (1,186)	(5)
Restricted stock award plan, shares		416,000	(143,000)
Employee stock purchase plan	362	$ 1		361
Employee stock purchase plan, shares		37,000
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 (loss)	55,181					55,181
Foreign currency translation	(100)				(100)
Unrealized gain (loss) on investments	(34)				(34)
COMPREHENSIVE INCOME (LOSS)	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
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	$ 852	$ (220,700)	628,295	(298)	(52,946)
BALANCE, shares at Dec. 31, 2018	85,173,686	85,174,000
BALANCE, shares at Dec. 31, 2018	(15,400,776)		(15,401,000)
Net income (loss)	$ 69,982					69,982
Foreign currency translation	(41)				(41)
Unrealized gain (loss) on investments	683				683
COMPREHENSIVE INCOME (LOSS)	70,624
Adjustment for change in accounting method	1,035					1,035
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
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)