CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		114,717
Beginning balance at Dec. 31, 2017	$ 190,968	$ 115	$ 384,074	$ 0	$ (75)	$ (193,146)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)		479
Issuance of common stock upon exercise of options	1,922		1,922
Stock-based compensation related to employee and director options	4,954		4,954
Net unrealized gain on marketable securities, net of tax	(77)				(77)
Net income	17,459					17,459
Ending balance (in shares) at Mar. 31, 2018		115,196
Ending balance at Mar. 31, 2018	215,226	$ 115	390,950	0	(152)	(175,687)
Beginning balance (in shares) at Dec. 31, 2017		114,717
Beginning balance at Dec. 31, 2017	190,968	$ 115	384,074	0	(75)	(193,146)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)		1,300
Net income	35,655
Ending balance (in shares) at Jun. 30, 2018		116,017
Ending balance at Jun. 30, 2018	243,507	$ 116	401,010	0	(128)	(157,491)
Beginning balance (in shares) at Mar. 31, 2018		115,196
Beginning balance at Mar. 31, 2018	215,226	$ 115	390,950	0	(152)	(175,687)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)		821
Issuance of common stock upon exercise of options	4,044	$ 1	4,043
Stock-based compensation related to employee and director options	6,017		6,017
Net unrealized gain on marketable securities, net of tax	24				24
Net income	18,196					18,196
Ending balance (in shares) at Jun. 30, 2018		116,017
Ending balance at Jun. 30, 2018	243,507	$ 116	401,010	0	(128)	(157,491)
Beginning balance (in shares) at Dec. 31, 2018		115,031
Beginning balance at Dec. 31, 2018	275,882	$ 117	417,228	(23,657)	(70)	(117,736)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)		1,497
Issuance of common stock upon exercise of options	3,366	$ 1	3,365
Shares tendered to satisfy cost and statutory withholding requirements for net settlement of cashless option exercise (in shares)		(428)
Shares tendered to satisfy cost and statutory withholding requirements for net settlement of cashless option exercise	(5,100)			(5,100)
Stock-based compensation related to employee and director options	6,724		6,724
Net unrealized gain on marketable securities, net of tax	164				164
Purchases of treasury stock (in shares)		(1,168)
Purchases of treasury stock	(13,555)			(13,555)
Net income	18,274					18,274
Ending balance (in shares) at Mar. 31, 2019		114,932
Ending balance at Mar. 31, 2019	285,755	$ 118	427,317	(42,312)	94	(99,462)
Beginning balance (in shares) at Dec. 31, 2018		115,031
Beginning balance at Dec. 31, 2018	275,882	$ 117	417,228	(23,657)	(70)	(117,736)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)		1,800
Net income	38,460
Ending balance (in shares) at Jun. 30, 2019		113,637
Ending balance at Jun. 30, 2019	298,053	$ 118	436,622	(59,732)	321	(79,276)
Beginning balance (in shares) at Mar. 31, 2019		114,932
Beginning balance at Mar. 31, 2019	285,755	$ 118	427,317	(42,312)	94	(99,462)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of options (in shares)		317
Issuance of common stock upon exercise of options	1,514	$ 0	1,514
Stock-based compensation related to employee and director options	7,791		7,791
Net unrealized gain on marketable securities, net of tax	227				227
Purchases of treasury stock (in shares)		(1,612)
Purchases of treasury stock	(17,420)			(17,420)
Net income	20,186					20,186
Ending balance (in shares) at Jun. 30, 2019		113,637
Ending balance at Jun. 30, 2019	$ 298,053	$ 118	$ 436,622	$ (59,732)	$ 321	$ (79,276)