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, 2014		54,486				0
Balances at period start at Dec. 31, 2014	$ 206,368	$ 5	$ 575,438	$ (16)	$ (369,059)	$ 0
Increase (Decrease) in Stockholders' Equity
Proceeds from public offering of common stock, net of issuance costs (in shares)		8,337
Proceeds from public offering of common stock, net of issuance costs	148,439	$ 1	148,438
Net proceeds from issuance of common stock pursuant to the ATM facility and settlement of milestone payment (in shares)		4,303
Net proceeds from issuance of common stock pursuant to the ATM facilities and settlement of milestone payment	64,503	$ 1	64,502
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)		1,846
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	24,567		24,567
Repurchase of common stock pursuant to share surrender (in shares)						(119)
Repurchase of common stock pursuant to share surrender	(2,048)					$ (2,048)
Issuance of restricted stock (in shares)		255
Cancellation/forfeiture of restricted stock (in shares)		(150)
Share-based compensation expense for employees	11,189		11,189
Share-based compensation expense for non-employees	251		251
Unrealized gain on marketable securities	20			20
Net loss	(83,313)				(83,313)
Balances at period end (in shares) at Dec. 31, 2015		69,077				(119)
Balances at period end at Dec. 31, 2015	369,976	$ 7	824,385	4	(452,372)	$ (2,048)
Increase (Decrease) in Stockholders' Equity
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
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
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 for employees	18,142		18,142
Share-based compensation expense for non-employees	180		180
Unrealized gain on marketable securities	82			82
Net loss	$ (21,003)				(21,003)
Balances at period end (in shares) at Dec. 31, 2016	71,076	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 ASU 2016-09	0		783		(783)
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 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
Issuance of restricted stock (in shares)		145
Cancellation/forfeiture of restricted stock (in shares)		(306)
Share-based compensation expense for employees	16,127		16,127
Unrealized gain 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)