Warrants to Acquire Shares of Common Stock - Warrants Outstanding (Details) shares in Thousands	3 Months Ended
	Mar. 31, 2022 $ / shares shares
Class of Warrant or Right, Outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	519
Canceled/Expired (in shares)	(1)
Outstanding, end of period (in shares)	518
Common stock warrants
Class of Warrant or Right, Outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	505
Canceled/Expired (in shares)	0
Outstanding, end of period (in shares)	505
January 2020 Offering | Common stock warrants
Class of Warrant or Right, Outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	309
Canceled/Expired (in shares)	0
Outstanding, end of period (in shares)	309
Exercise price (in dollars per share) | $ / shares	$ 3.93
July 2020 PIPE Offering | Common stock warrants
Class of Warrant or Right, Outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	25
Canceled/Expired (in shares)	0
Outstanding, end of period (in shares)	25
Exercise price (in dollars per share) | $ / shares	$ 3.30
July 2018 Offering | Common stock warrants
Class of Warrant or Right, Outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	132
Canceled/Expired (in shares)	0
Outstanding, end of period (in shares)	132
Exercise price (in dollars per share) | $ / shares	$ 7.50
March 2019 Exercise Agreement | Common stock warrants
Class of Warrant or Right, Outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	30
Canceled/Expired (in shares)	0
Outstanding, end of period (in shares)	30
Exercise price (in dollars per share) | $ / shares	$ 7.50
Other | Common stock warrants
Class of Warrant or Right, Outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	9
Canceled/Expired (in shares)	0
Outstanding, end of period (in shares)	9
Exercise price (in dollars per share) | $ / shares	$ 306.66
Warrants classified as liability
Class of Warrant or Right, Outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	14
Canceled/Expired (in shares)	(1)
Outstanding, end of period (in shares)	13
Exercise price (in dollars per share) | $ / shares	$ 7.50