Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
BALANCE at Dec. 31, 2014	$ 247,653	$ 4	$ 926,769	$ (95)	$ (679,025)
BALANCE (in shares) at Dec. 31, 2014		41,312,000
Exercise of options for common stock	$ 10,010		10,010
Exercise of options for common stock, shares	816,696	817,000
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		181,000
Shares withheld for taxes	$ (182)		(182)
Shares withheld for taxes, shares		(6,000)
Issuance of common stock for cash, net of offering costs	119,916	$ 1	119,915
Issuance of common stock for cash, net of offering costs, shares		3,250,000
Issuance of common stock under employee stock purchase plan	879		879
Issuance of common stock under employee stock purchase plan, shares		76,000
Stock-based compensation	32,117		32,117
Unrealized loss from available-for-sale securities	(16)			(16)
Net loss	(220,030)				(220,030)
BALANCE at Dec. 31, 2015	190,347	$ 5	1,089,508	(111)	(899,055)
BALANCE (in shares) at Dec. 31, 2015		45,630,000
Exercise of options for common stock	$ 19,353		19,353
Exercise of options for common stock, shares	1,056,821	1,113,000
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		50,000
Shares withheld for taxes	$ (2,168)		(2,168)
Shares withheld for taxes, shares		(47,000)
Issuance of common stock for cash, net of offering costs	364,749		364,749
Issuance of common stock for cash, net of offering costs, shares		7,876,000
Issuance of common stock under employee stock purchase plan	1,577		1,577
Issuance of common stock under employee stock purchase plan, shares		137,000
Stock-based compensation	30,107		30,107
Unrealized loss from available-for-sale securities	(9)			(9)
Net loss	(267,265)				(267,265)
BALANCE at Dec. 31, 2016	336,691	$ 5	1,503,126	(120)	(1,166,320)
BALANCE (in shares) at Dec. 31, 2016		54,759,000
Exercise of options for common stock	$ 13,799		13,799
Exercise of options for common stock, shares	792,845	793,000
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		400,000
Shares withheld for taxes	$ (2,227)		(2,227)
Shares withheld for taxes, shares		(60,000)
Issuance of common stock for cash, net of offering costs	353,959	$ 1	353,958
Issuance of common stock for cash, net of offering costs, shares		8,798,000
Issuance of common stock under employee stock purchase plan	1,425		1,425
Issuance of common stock under employee stock purchase plan, shares		102,000
Equity component of convertible notes	156,953		156,953
Purchase of capped call share options	(50,901)		(50,901)
Stock-based compensation	30,465		30,465
Unrealized loss from available-for-sale securities	(259)			(259)
Net loss	(50,688)				(50,688)
BALANCE at Dec. 31, 2017	$ 789,217	$ 6	$ 2,006,598	$ (379)	$ (1,217,008)
BALANCE (in shares) at Dec. 31, 2017		64,792,000