Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	AOCI Attributable to Parent [Member]	Accumulated Deficit	Total
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 (in shares)	(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
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
Increase (Decrease) in Stockholders' Equity
Net Income (Loss) Available to Common Stockholders, Basic					$ (90,970)
Balance (in shares) at Mar. 31, 2016					4,404,658
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
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs	$ 1	838,090			$ 838,091
Issuance of common stock, net of issuance costs (in shares)	12,632,356
Issuance of common stock resulting from exercise of stock options		25,958			25,958
Issuance of common stock resulting from exercise of stock options (in shares)	643,035
Issuance of common stock resulting from Employee Stock Purchase Plan		2,229			2,229
Issuance of common stock resulting from Employee Stock Purchase Plan (in shares)	42,478
Issuance of common stock resulting from the vesting of restricted stock units, net of shares withheld for taxes (in shares)	18,000
Issuance of common stock as payment of Board of Directors fees in lieu of cash		345			345
Issuance of common stock as payment of Board of Directors fees in lieu of cash (in shares)	6,027
Stock-based compensation expense		48,110			48,110
Other comprehensive income/(loss)			$ (2,924)		(2,924)
Net Income (Loss) Available to Common Stockholders, Basic				(387,472)	(387,472)
Balance at Dec. 31, 2016	$ 5	1,604,798	$ (2,924)	(990,668)	$ 611,211
Balance (in shares) at Dec. 31, 2016	53,621,679				53,621,679
Increase (Decrease) in Stockholders' Equity
Retroactive adjustment to beginning accumulated deficit and additional paid-in capital resulting from adoption of ASU 2016-09		$ 1,278		$ (1,278)