Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2016		33,458,607
Beginning balance, value at Dec. 31, 2016	$ 105,344	$ 33	$ 422,002	$ (68)	$ (316,623)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of commissions and expenses (in shares)		2,506,387
Issuance of common stock, net of discounts, commissions and expenses	133,812	$ 2	133,810
Issuance of common stock upon exercise of stock purchase rights (in shares)		27,953
Issuance of common stock upon exercise of stock purchase rights	1,050		1,050
Issuance of common stock upon exercise of stock options and warrants (in shares)		201,592
Issuance of common stock upon exercise of stock options and warrants	828	$ 1	827
Issuance of common stock for restricted stock awards, net (in shares)		489,952
Issuance of common stock for restricted stock awards, net	(750)	$ 1	(751)
Shares issued for asset acquisition (in shares)		263,146
Shares issued for asset acquisition	14,302		14,302
Stock-based compensation	26,078		26,078
Other comprehensive income	40			40
Net loss	(145,105)				(145,105)
Ending balance (in shares) at Dec. 31, 2017		36,947,637
Ending balance, value at Dec. 31, 2017	135,599	$ 37	597,318	(28)	(461,728)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of commissions and expenses (in shares)		2,313,824
Issuance of common stock, net of discounts, commissions and expenses	136,445	$ 2	136,443
Issuance of common stock upon exercise of stock purchase rights (in shares)		34,193
Issuance of common stock upon exercise of stock purchase rights	1,401		1,401
Issuance of common stock upon exercise of stock options and warrants (in shares)		597,777
Issuance of common stock upon exercise of stock options and warrants	4,251	$ 1	4,250
Issuance of common stock for restricted stock awards, net (in shares)		216,005
Issuance of common stock for restricted stock awards, net	(2,172)		(2,172)
Issuance of shares upon conversion of 2014 Convertible Notes (in shares)		5,369,447
Issuance of shares upon conversion of 2014 Convertible Notes	148,083	$ 5	148,078
Stock-based compensation	38,862		38,862
Other comprehensive income	28			28
Net loss	(232,569)				(232,569)
Ending balance (in shares) at Dec. 31, 2018		45,478,883
Ending balance, value at Dec. 31, 2018	227,806	$ 45	924,180	0	(696,419)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock purchase rights (in shares)		42,611
Issuance of common stock upon exercise of stock purchase rights	$ 979		979
Issuance of common stock upon exercise of stock options and warrants (in shares)	287,148	612,759
Issuance of common stock upon exercise of stock options and warrants	$ 3,140		3,140
Issuance of common stock for restricted stock awards, net (in shares)		330,416
Issuance of common stock for restricted stock awards, net	(2,629)	$ 1	(2,630)
Stock-based compensation	45,551		45,551
Other comprehensive income	(92)			(92)
Equity component of Convertible Notes, net of issuance costs of $3,725	124,666		124,666
Payment for capped call share options	(32,890)		(32,890)
Net loss	(199,581)				(199,581)
Ending balance (in shares) at Dec. 31, 2019		46,464,669
Ending balance, value at Dec. 31, 2019	$ 166,950	$ 46	$ 1,062,996	$ (92)	$ (896,000)