Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	2019 Convertible Senior Notes	2023 Convertible Senior Notes	Common Stock	Additional paid in capital	Additional paid in capital 2019 Convertible Senior Notes	Additional paid in capital 2023 Convertible Senior Notes	Accumulated other comprehensive income (loss)	Retain earnings (Accumulated deficit)
Balance at beginning of period (shares) at Dec. 31, 2017				21,149,000
Balance at beginning of period at Dec. 31, 2017	$ 399,788			$ 21	$ 798,205			$ 2,486	$ (400,924)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under employee stock compensation plans, net (shares)				166,000
Issuance of common stock under employee stock compensation plans, net	5,119			$ 1	5,118
Reclassification of equity component of currently redeemable convertible notes	2,781				2,781
Share-based compensation	4,555				4,555
Repurchase of common stock (shares)				(13,000)
Repurchase of common stock	(1,895)			$ (1)	(1,894)
Unrealized gain (loss) on available-for-sale securities, net of deferred tax	(110)							(110)
Net income	45,279								45,279
Balance at end of period (shares) at Mar. 31, 2018				21,302,000
Balance at end of period at Mar. 31, 2018	481,100			$ 21	808,765			(286)	(327,400)
Balance at beginning of period (shares) at Dec. 31, 2017				21,149,000
Balance at beginning of period at Dec. 31, 2017	399,788			$ 21	798,205			2,486	(400,924)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	185,803
Balance at end of period (shares) at Sep. 30, 2018				21,226,000
Balance at end of period at Sep. 30, 2018	699,520			$ 21	883,477			(64)	(183,914)
Balance at beginning of period (shares) at Mar. 31, 2018				21,302,000
Balance at beginning of period at Mar. 31, 2018	481,100			$ 21	808,765			(286)	(327,400)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under employee stock compensation plans, net (shares)				60,000
Issuance of common stock under employee stock compensation plans, net	3,296				3,296
Reclassification of equity component of currently redeemable convertible notes	16,078				16,078
Share-based compensation	4,812				4,812
Repurchase of common stock (shares)				(267,000)
Repurchase of common stock	(50,832)				(50,832)
Unrealized gain (loss) on available-for-sale securities, net of deferred tax	(495)							(495)
Derivative associated with Notes and Bond Hedge		$ (1,559)	$ (1,807)			$ (1,559)	$ (1,807)
Loss on settlement of Notes		590				590
Tax effect on Notes transactions		$ 67	(3,240)			$ 67	(3,240)
Value of warrants issued			$ 97,805				$ 97,805
Other tax adjustments	838				208			630
Net income	73,160								73,160
Balance at end of period (shares) at Jun. 30, 2018				21,095,000
Balance at end of period at Jun. 30, 2018	619,813			$ 21	874,183			(151)	(254,240)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under employee stock compensation plans, net (shares)				131,000
Issuance of common stock under employee stock compensation plans, net	6,788				6,788
Share-based compensation	5,470				5,470
Other comprehensive income	87							87
Other tax adjustments	0				(2,964)				2,964
Net income	67,362								67,362
Balance at end of period (shares) at Sep. 30, 2018				21,226,000
Balance at end of period at Sep. 30, 2018	699,520			$ 21	883,477			(64)	(183,914)
Balance at beginning of period (shares) at Dec. 31, 2018				20,765,000
Balance at beginning of period at Dec. 31, 2018	560,914			$ 21	791,114			(1,024)	(229,197)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under employee stock compensation plans, net (shares)				135,000
Issuance of common stock under employee stock compensation plans, net	(991)				(991)
Share-based compensation	5,347				5,347
Repurchase of common stock (shares)				(1,236,000)
Repurchase of common stock	(151,585)			$ (1)	(151,584)
Unrealized gain (loss) on available-for-sale securities, net of deferred tax	230							230
Foreign currency translation adjustment	291							291
Other tax adjustments	(569)				(569)
Net income	666,337								666,337
Balance at end of period (shares) at Mar. 31, 2019				19,664,000
Balance at end of period at Mar. 31, 2019	1,079,974			$ 20	643,317			(503)	437,140
Balance at beginning of period (shares) at Dec. 31, 2018				20,765,000
Balance at beginning of period at Dec. 31, 2018	560,914			$ 21	791,114			(1,024)	(229,197)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of common stock	(366,500)
Net income	636,667
Balance at end of period (shares) at Sep. 30, 2019				17,563,000
Balance at end of period at Sep. 30, 2019	850,581			$ 17	444,587			(1,493)	407,470
Balance at beginning of period (shares) at Mar. 31, 2019				19,664,000
Balance at beginning of period at Mar. 31, 2019	1,079,974			$ 20	643,317			(503)	437,140
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under employee stock compensation plans, net (shares)				17,000
Issuance of common stock under employee stock compensation plans, net	740				740
Share-based compensation	6,571				6,571
Repurchase of common stock (shares)				(291,000)
Repurchase of common stock	(33,717)			$ (1)	(33,716)
Unrealized gain (loss) on available-for-sale securities, net of deferred tax	503							503
Foreign currency translation adjustment	(542)							(542)
Other tax adjustments	2,343				2,343
Net income	(14,419)								(14,419)
Balance at end of period (shares) at Jun. 30, 2019				19,390,000
Balance at end of period at Jun. 30, 2019	1,041,453			$ 19	619,255			(542)	422,721
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under employee stock compensation plans, net (shares)				7,000
Issuance of common stock under employee stock compensation plans, net	199				199
Share-based compensation	6,297				6,297
Repurchase of common stock (shares)				(1,834,000)
Repurchase of common stock	(181,188)			$ (2)	(181,186)
Unrealized gain (loss) on available-for-sale securities, net of deferred tax	(187)							(187)
Foreign currency translation adjustment	(764)							(764)
Other tax adjustments	22				22
Net income	(15,251)								(15,251)
Balance at end of period (shares) at Sep. 30, 2019				17,563,000
Balance at end of period at Sep. 30, 2019	$ 850,581			$ 17	$ 444,587			$ (1,493)	$ 407,470