CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS’ EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	2.50% Convertible Notes due 2021	Convertible debt 2.50% Convertible Notes due 2021	Convertible debt 5.00% Convertible Notes due 2024	Common Stock	Additional Paid-In Capital	Additional Paid-In Capital Convertible debt 2.50% Convertible Notes due 2021	Additional Paid-In Capital Convertible debt 5.00% Convertible Notes due 2024	Accumulated Earnings (Deficit)	Accumulated Other Comprehensive Loss
Balances (in shares) at Dec. 31, 2017					63,400
Balances at Dec. 31, 2017	$ 169,508				$ 6	$ 389,015			$ (219,508)	$ (5)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)					120
Issuance of common stock upon exercise of options	636					636
Issuance of common stock in conjunction with vesting of restricted stock units (in shares)					33
Stock-based compensation	2,234					2,234
Shares withheld for payment of employee's withholding tax liability	(114)					(114)
Unrealized loss (gain) on available-for-sale securities	2									2
Net income (loss)	33,824								33,824
Balances (in shares) at Mar. 31, 2018					63,553
Balances at Mar. 31, 2018	206,090				$ 6	391,771			(185,684)	(3)
Balances (in shares) at Dec. 31, 2017					63,400
Balances at Dec. 31, 2017	169,508				$ 6	389,015			(219,508)	(5)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	61,046
Balances (in shares) at Sep. 30, 2018					63,939
Balances at Sep. 30, 2018	242,409				$ 6	400,869			(158,462)	(4)
Balances (in shares) at Mar. 31, 2018					63,553
Balances at Mar. 31, 2018	206,090				$ 6	391,771			(185,684)	(3)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)					108
Issuance of common stock upon exercise of options	667					667
Issuance of common stock under employee stock purchase plan (in shares)					72
Issuance of common stock under employee stock purchase plan	381					381
Issuance of common stock in conjunction with vesting of restricted stock units (in shares)					169
Stock-based compensation	5,270					5,270
Shares withheld for payment of employee's withholding tax liability	(153)					(153)
Unrealized loss (gain) on available-for-sale securities	(1)									(1)
Net income (loss)	(21,048)								(21,048)
Balances (in shares) at Jun. 30, 2018					63,902
Balances at Jun. 30, 2018	191,206				$ 6	397,936			(206,732)	(4)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)					37
Issuance of common stock upon exercise of options	166					166
Issuance of common stock in conjunction with vesting of restricted stock units	(4)					(4)
Stock-based compensation	2,771					2,771
Net income (loss)	48,270								48,270
Balances (in shares) at Sep. 30, 2018					63,939
Balances at Sep. 30, 2018	242,409				$ 6	400,869			(158,462)	(4)
Balances (in shares) at Dec. 31, 2018					64,185
Balances at Dec. 31, 2018	220,335				$ 6	402,934			(182,600)	(5)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)					14
Issuance of common stock upon exercise of options	25					25
Issuance of common stock in conjunction with vesting of restricted stock units (in shares)					132
Stock-based compensation	2,702					2,702
Shares withheld for payment of employee's withholding tax liability	(216)					(216)
Net income (loss)	(14,301)								(14,301)
Balances (in shares) at Mar. 31, 2019					64,331
Balances at Mar. 31, 2019	208,545				$ 6	405,445			(196,901)	(5)
Balances (in shares) at Dec. 31, 2018					64,185
Balances at Dec. 31, 2018	220,335				$ 6	402,934			(182,600)	(5)
Increase (Decrease) in Stockholders' Equity
Net income (loss)	(24,575)
Balances (in shares) at Sep. 30, 2019					80,680
Balances at Sep. 30, 2019	248,434				$ 8	455,601			(207,175)	0
Balances (in shares) at Mar. 31, 2019					64,331
Balances at Mar. 31, 2019	208,545				$ 6	405,445			(196,901)	(5)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under employee stock purchase plan (in shares)					64
Issuance of common stock under employee stock purchase plan	158					158
Issuance of common stock in conjunction with vesting of restricted stock units (in shares)					426
Stock-based compensation	2,634					2,634
Shares withheld for payment of employee's withholding tax liability	(293)					(293)
Net income (loss)	(13,605)								(13,605)
Balances (in shares) at Jun. 30, 2019					64,821
Balances at Jun. 30, 2019	197,439				$ 6	407,944			(210,506)	(5)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in conjunction with vesting of restricted stock units (in shares)					42
Issuance of common stock in conjunction with the Convertible Note Exchange (in shares)					15,817
Issuance of common stock in conjunction with the Convertible Note Exchange	25,307				$ 2	25,305
Reacquisition of equity component of 2021 Notes, net of tax		$ 6,200	$ (4,796)	$ 24,165			$ (4,796)	$ 24,165
Equity component of issued 2024 Notes, net of tax		$ 6,200	$ (4,796)	$ 24,165			$ (4,796)	$ 24,165
Stock-based compensation	3,004					3,004
Shares withheld for payment of employee's withholding tax liability	(19)					(19)
Unrealized loss (gain) on available-for-sale securities	5									5
Net income (loss)	3,331								3,331
Balances (in shares) at Sep. 30, 2019					80,680
Balances at Sep. 30, 2019	$ 248,434				$ 8	$ 455,601			$ (207,175)	$ 0