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	41,110					41,110
Foreign currency translation	(103)				(103)
Unrealized gain (loss) on investments	(4)				(4)
Share-based compensation expense	4,144			4,144
Common stock issued	(1,705)	$ 9	$ (3,313)	1,599
Common stock issued, shares		879,000	(236,000)
BALANCE at Sep. 30, 2018	339,647	$ 852	$ (220,668)	626,751	(271)	(67,017)
BALANCE, shares at Sep. 30, 2018		85,159,000
BALANCE, shares at Sep. 30, 2018			(15,398,000)
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)
Net income	14,857					14,857
Foreign currency translation	(21)				(21)
Unrealized gain (loss) on investments	106				106
Share-based compensation expense	1,445			1,445
Common stock issued	343	$ 1	$ (95)	437
Common stock issued, shares		44,000	(6,000)
BALANCE at Sep. 30, 2018	339,647	$ 852	$ (220,668)	626,751	(271)	(67,017)
BALANCE, shares at Sep. 30, 2018		85,159,000
BALANCE, shares at Sep. 30, 2018			(15,398,000)
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	$ 42,466					42,466
Foreign currency translation	(130)				(130)
Unrealized gain (loss) on investments	814				814
Adjustment for change in accounting method	1,035					1,035
Share-based compensation expense	3,922			3,922
Common stock issued	(1,366)	$ 7	$ (2,692)	1,319
Common stock issued, shares		703,000	(163,000)
BALANCE at Sep. 30, 2019	$ 401,944	$ 859	$ (223,392)	633,536	386	(9,445)
BALANCE, shares at Sep. 30, 2019	85,876,510	85,877,000
BALANCE, shares at Sep. 30, 2019	(15,564,086)		(15,564,000)
BALANCE at Jun. 30, 2019	$ 381,248	$ 857	$ (223,263)	630,878	455	(27,679)
BALANCE, shares at Jun. 30, 2019		85,668,000
BALANCE, shares at Jun. 30, 2019			(15,558,000)
Net income	18,234					18,234
Foreign currency translation	(113)				(113)
Unrealized gain (loss) on investments	44				44
Share-based compensation expense	1,516			1,516
Common stock issued	1,015	$ 2	$ (129)	1,142
Common stock issued, shares		209,000	(6,000)
BALANCE at Sep. 30, 2019	$ 401,944	$ 859	$ (223,392)	$ 633,536	$ 386	$ (9,445)
BALANCE, shares at Sep. 30, 2019	85,876,510	85,877,000
BALANCE, shares at Sep. 30, 2019	(15,564,086)		(15,564,000)