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	17,502					17,502
Foreign currency translation	86				86
Unrealized gain (loss) on investments	(218)				(218)
Share-based compensation expense	1,501			1,501
Common stock issued	(2,105)	$ 7	$ (2,981)	869
Common stock issued, shares		708,000	(215,000)
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)
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	$ 24,791					24,791
Foreign currency translation	(52)				(52)
Unrealized gain (loss) on investments	399				399
Adjustment for change in accounting method	1,035					1,035
Share-based compensation expense	1,369			1,369
Common stock issued	(2,423)	$ 5	$ (2,532)	104
Common stock issued, shares		485,000	(155,000)
BALANCE at Mar. 31, 2019	$ 380,322	$ 857	$ (223,232)	$ 629,768	$ 49	$ (27,120)
BALANCE, shares at Mar. 31, 2019	85,658,822	85,659,000
BALANCE, shares at Mar. 31, 2019	(15,555,913)		(15,556,000)