CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit	Treasury Stock	Total
Balances at Dec. 31, 2012	$ 5,000	$ 522,422,000	$ 111,000	$ (162,051,000)		$ 360,487,000
Balances (in shares) at Dec. 31, 2012	51,709
Increase (Decrease) in Stockholders' Equity
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan		5,022,000				5,022,000
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	516
Issuance of restricted stock (in shares)	172
Cancellation/forfeiture of restricted stock	(40)
Share-based compensation expense for employees		12,668,000				12,668,000
Share-based compensation expense for non-employees		154,000				154,000
Unrealized gain on marketable securities			(86,000)			(86,000)
Net loss				(108,408,000)		(108,408,000)
Balances at Dec. 31, 2013	$ 5,000	540,266,000	25,000	(270,459,000)		269,837,000
Balances (in shares) at Dec. 31, 2013	52,357
Increase (Decrease) in Stockholders' Equity
Net proceeds from issuance of common stock pursuant to the ATM facility		18,305,000				18,305,000
Net proceeds from issuance of common stock pursuant to the ATM facility (in shares)	1,612
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan		3,273,000				3,273,000
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	332
Issuance of restricted stock (in shares)	227
Cancellation/forfeiture of restricted stock	(42)
Share-based compensation expense for employees		13,562,000				13,562,000
Share-based compensation expense for non-employees		32,000				32,000
Unrealized gain on marketable securities			(41,000)			(41,000)
Net loss				(98,600,000)		(98,600,000)
Balances at Dec. 31, 2014	$ 5,000	575,438,000	(16,000)	(369,059,000)		$ 206,368,000
Balances (in shares) at Dec. 31, 2014	54,486					54,486
Increase (Decrease) in Stockholders' Equity
Proceeds from public offering of common stock, net of issuance costs	$ 1,000	148,438,000				$ 148,439,000
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 facility	$ 1,000	64,502,000				64,503,000
Net proceeds from issuance of common stock pursuant to the ATM facility (in shares)	4,303
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan		24,567,000				24,567,000
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	1,846
Repurchase of common stock pursuant to share surrender					$ (2,048,000)	(2,048,000)
Repurchase of common stock pursuant to share surrender					119
Issuance of restricted stock (in shares)	255
Cancellation/forfeiture of restricted stock	(150)
Share-based compensation expense for employees		11,189,000				11,189,000
Share-based compensation expense for non-employees		251,000				251,000
Unrealized gain on marketable securities			20,000.000			20,000
Net loss				(83,313,000.000)		(83,313,000)
Balances at Dec. 31, 2015	$ 7,000	$ 824,385,000	$ 4,000	$ (452,372,000)	$ (2,048,000)	$ 369,976,000
Balances (in shares) at Dec. 31, 2015	69,077				(119)	69,077