CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Treasury Stock
Balances at period start (in shares) at Dec. 31, 2015		69,077				(119)
Balances at period start at Dec. 31, 2015	$ 369,976	$ 7	$ 824,385	$ 4	$ (452,372)	$ (2,048)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)		211
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	2,407		2,407
Net proceeds from issuance of common stock pursuant to the ATM facility and settlement of milestone payment (in shares)		266
Net proceeds from issuance of common stock pursuant to the ATM facilities and settlement of milestone payment	3,190		3,190
Repurchase of common stock pursuant to share surrender (in shares)						(110)
Repurchase of common stock pursuant to share surrender	(1,066)					$ (1,066)
Issuance of restricted stock (in shares)		2,081
Cancellation/forfeiture of restricted stock (in shares)		(330)
Share-based compensation expense	18,142		18,142
Share-based compensation expense for non-employees	180		180
Unrealized loss on marketable securities	82			82
Net loss	(21,003)				(21,003)
Balances at period end (in shares) at Dec. 31, 2016		71,305				(229)
Balances at period end at Dec. 31, 2016	371,908	$ 7	848,304	86	(473,375)	$ (3,114)
Increase (Decrease) in Stockholders' Equity
Impact of adopting ASC 606			783		(783)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)		903
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	10,351		10,351
Net proceeds from issuance of common stock pursuant to the ATM facility and settlement of milestone payment (in shares)		4,537
Net proceeds from issuance of common stock pursuant to the ATM facilities and settlement of milestone payment	64,090	$ 1	64,089
Issuance of restricted stock (in shares)		145
Cancellation/forfeiture of restricted stock (in shares)		(306)
Share-based compensation expense	16,127		16,127
Unrealized loss on marketable securities	(226)			(226)
Net loss	$ (88,096)				(88,096)
Balances at period end (in shares) at Dec. 31, 2017	76,355	76,584				(229)
Balances at period end at Dec. 31, 2017	$ 374,154	$ 8	939,654	(140)	(562,254)	$ (3,114)
Increase (Decrease) in Stockholders' Equity
Impact of adopting ASC 606	(5,511)				(5,511)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)		2,080
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	29,365		29,365
Net proceeds from issuance of common stock pursuant to the ATM facility and settlement of milestone payment (in shares)		20,000
Net proceeds from issuance of common stock pursuant to the ATM facilities and settlement of milestone payment	217,786	$ 2
Issuance of restricted stock (in shares)		445
Cancellation/forfeiture of restricted stock (in shares)		(414)
Share-based compensation expense	21,222		21,222
Unrealized loss on marketable securities	53			53
Net loss	$ (176,061)				(176,061)
Balances at period end (in shares) at Dec. 31, 2018	98,464	98,695				(229)
Balances at period end at Dec. 31, 2018	$ 461,008	$ 10	$ 1,208,025	$ (87)	$ (743,826)	$ (3,114)