Consolidated Statements of Convertible Preferred Stock and Stockholders' (Deficit) Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Total
Balance at Dec. 31, 2012	$ 3	$ 202,795	$ (87,136)	$ 115,662
Balance (in shares) at Dec. 31, 2012	27,136,329
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs		91,312		91,312
Issuance of common stock, net of issuance costs (in shares)	5,428,000
Issuance of common stock resulting from exercise of stock options		634		634
Issuance of common stock resulting from exercise of stock options (in shares)	162,480
Issuance of common stock resulting from Employee Stock Purchase Plan		147		147
Issuance of common stock resulting from Employee Stock Purchase Plan (in shares)	7,831
Issuance of common stock as payment of Board of Directors fees in lieu of cash		253		253
Issuance of common stock as payment of Board of Directors fees in lieu of cash (in shares)	4,368
Stock-based compensation expense		7,506		7,506
Net Income (Loss) Available to Common Stockholders, Basic			(92,362)	(92,362)
Balance at Dec. 31, 2013	$ 3	302,647	(179,498)	123,152
Balance (in shares) at Dec. 31, 2013	32,739,008
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs	$ 1	94,198		94,199
Issuance of common stock, net of issuance costs (in shares)	3,200,000
Issuance of common stock resulting from exercise of stock options		1,630		1,630
Issuance of common stock resulting from exercise of stock options (in shares)	159,747
Issuance of common stock resulting from Employee Stock Purchase Plan		249		249
Issuance of common stock resulting from Employee Stock Purchase Plan (in shares)	9,845
Cancellation of unvested restricted stock	(8,929)
Issuance of common stock as payment of Board of Directors fees in lieu of cash		274		274
Issuance of common stock as payment of Board of Directors fees in lieu of cash (in shares)	10,411
Conversion option of convertible notes - net of issuance costs		84,984		84,984
Capped call options associated with convertible notes		(20,829)		(20,829)
Stock-based compensation expense		11,409		11,409
Net Income (Loss) Available to Common Stockholders, Basic			(171,012)	(171,012)
Balance at Dec. 31, 2014	$ 4	474,562	(350,510)	$ 124,056
Balance (in shares) at Dec. 31, 2014	36,110,082			36,110,082
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs		179,753		$ 179,753
Issuance of common stock, net of issuance costs (in shares)	3,755,000
Issuance of common stock resulting from exercise of stock options		7,930		7,930
Issuance of common stock resulting from exercise of stock options (in shares)	382,580
Issuance of common stock resulting from Employee Stock Purchase Plan		830		830
Issuance of common stock resulting from Employee Stock Purchase Plan (in shares)	20,601
Issuance of common stock resulting from the vesting of restricted stock units, net of shares withheld for taxes		(201)		(201)
Issuance of common stock resulting from the vesting of restricted stock units, net of shares withheld for taxes (in shares)	8,091
Issuance of common stock as payment of Board of Directors fees in lieu of cash		266		266
Issuance of common stock as payment of Board of Directors fees in lieu of cash (in shares)	3,429
Stock-based compensation expense		25,648		25,648
Net Income (Loss) Available to Common Stockholders, Basic			(251,408)	(251,408)
Balance at Dec. 31, 2015	$ 4	$ 688,788	$ (601,918)	$ 86,874
Balance (in shares) at Dec. 31, 2015	40,279,783			40,279,783