CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit	Treasury Stock
Balances at period start at Dec. 31, 2013	$ 269,837	$ 5	$ 540,266	$ 25	$ (270,459)
Balances at period start (in shares) at Dec. 31, 2013		52,357
Increase (Decrease) in Stockholders' Equity
Net proceeds from issuance of common stock pursuant to the ATM facilities	18,305		18,305
Net proceeds from issuance of common stock pursuant to the ATM facility (in shares)		1,612
Issuance of common stock	3,273		3,273
Issuance of common stock (in shares)		332
Issuance of restricted stock (in shares)		227
Cancellation/forfeiture of restricted stock	0
Cancellation/forfeiture of restricted stock (in shares)		(42)
Share-based compensation expense for employees	13,562		13,562
Share-based compensation expense for non-employees	32		32
Unrealized gain on marketable securities	(41)			(41)	0
Net loss	(98,600)				(98,600)
Balances at period end at Dec. 31, 2014	206,368	$ 5	575,438	(16)	(369,059)
Balances at period end (in shares) at Dec. 31, 2014		54,486
Increase (Decrease) in Stockholders' Equity
Proceeds from public offering of common stock, net of issuance costs	148,439	$ 1	148,438
Proceeds from public offering of common stock, net of issuance costs (in shares)		8,337
Net proceeds from issuance of common stock pursuant to the ATM facilities	64,503	$ 1	64,502
Net proceeds from issuance of common stock pursuant to the ATM facility (in shares)		4,303
Issuance of common stock	24,567		24,567
Issuance of common stock (in shares)		1,846
Repurchase of common stock pursuant to share surrender	(2,048)					$ (2,048)
Repurchase of common stock pursuant to share surrender (in shares)		0				(119)
Issuance of restricted stock (in shares)		255
Cancellation/forfeiture of restricted stock	0
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	0
Unrealized gain on marketable securities	20			20
Net loss	(83,313)				(83,313)
Balances at period end at Dec. 31, 2015	$ 369,976	$ 7	824,385	4	(452,372)	$ (2,048)
Balances at period end (in shares) at Dec. 31, 2015	68,958	69,077				(119)
Increase (Decrease) in Stockholders' Equity
Net proceeds from issuance of common stock pursuant to the ATM facilities	$ 3,190		3,190
Net proceeds from issuance of common stock pursuant to the ATM facility (in shares)		266
Issuance of common stock	2,407		2,407
Issuance of common stock (in shares)		211
Repurchase of common stock pursuant to share surrender	(1,066)					$ (1,066)
Repurchase of common stock pursuant to share surrender (in shares)		0				(110)
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 at Dec. 31, 2016	$ 371,908	$ 7	$ 848,304	$ 86	$ (473,375)	$ (3,114)
Balances at period end (in shares) at Dec. 31, 2016	71,076	71,305				(229)