Unaudited Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Gain (Loss) [Member]	Accumulated Deficit [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 (loss)	26,253					26,253
Foreign currency translation	(82)				(82)
Unrealized gain (loss) on investments	(110)				(110)
Share-based compensation expense	2,699			2,699
Common stock issued	(2,048)	$ 8	$ (3,218)	1,162
Common stock issued, shares		835,000	(230,000)
BALANCE at Jun. 30, 2018	322,917	$ 851	$ (220,573)	624,869	(356)	(81,874)
BALANCE, shares at Jun. 30, 2018		85,115,000
BALANCE, shares at Jun. 30, 2018			(15,392,000)
BALANCE at Mar. 31, 2018	312,971	$ 850	$ (220,336)	623,378	(296)	(90,625)
BALANCE, shares at Mar. 31, 2018		84,988,000
BALANCE, shares at Mar. 31, 2018			(15,377,000)
Net income (loss)	8,751					8,751
Foreign currency translation	(168)				(168)
Unrealized gain (loss) on investments	108				108
Share-based compensation expense	1,198			1,198
Common stock issued	57	$ 1	$ (237)	293
Common stock issued, shares		127,000	(15,000)
BALANCE at Jun. 30, 2018	322,917	$ 851	$ (220,573)	624,869	(356)	(81,874)
BALANCE, shares at Jun. 30, 2018		85,115,000
BALANCE, shares at Jun. 30, 2018			(15,392,000)
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)	$ 24,232					24,232
Foreign currency translation	(17)				(17)
Unrealized gain (loss) on investments	770				770
Adjustment for change in accounting method	1,035					1,035
Share-based compensation expense	2,406			2,406
Common stock issued	(2,381)	$ 5	$ (2,563)	177
Common stock issued, shares		494,000	(157,000)
BALANCE at Jun. 30, 2019	$ 381,248	$ 857	$ (223,263)	630,878	455	(27,679)
BALANCE, shares at Jun. 30, 2019	85,668,036	85,668,000
BALANCE, shares at Jun. 30, 2019	(15,557,493)		(15,558,000)
BALANCE at Mar. 31, 2019	$ 380,322	$ 857	$ (223,232)	629,768	49	(27,120)
BALANCE, shares at Mar. 31, 2019		85,659,000
BALANCE, shares at Mar. 31, 2019			(15,556,000)
Net income (loss)	(559)					(559)
Foreign currency translation	35				35
Unrealized gain (loss) on investments	371				371
Share-based compensation expense	1,037			1,037
Common stock issued	42		$ (31)	73
Common stock issued, shares		9,000	(2,000)
BALANCE at Jun. 30, 2019	$ 381,248	$ 857	$ (223,263)	$ 630,878	$ 455	$ (27,679)
BALANCE, shares at Jun. 30, 2019	85,668,036	85,668,000
BALANCE, shares at Jun. 30, 2019	(15,557,493)		(15,558,000)