Stockholders' Equity - Common Stock (Details) - USD ($) $ / shares in Units, $ in Millions					1 Months Ended	3 Months Ended					9 Months Ended		18 Months Ended
	Jul. 13, 2020	Apr. 07, 2020	Dec. 23, 2019	Mar. 04, 2019	Nov. 30, 2018	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Dec. 31, 2020
Class of Stock
Stock issued (per share)				$ 2.84
Par value (per share)						$ 0.001					$ 0.001		$ 0.001	$ 0.001
Proceeds from issuance			$ 3.9
Warrant exercised price (per share)						$ 2.04					$ 2.04		$ 2.04	$ 1.28
"At-The-Market" Equity Program
Class of Stock
Value of shares which may be sold						$ 19.5					$ 19.5		$ 19.5
Stock issued (in shares)											5,117,357	938,669
Net proceeds from offering of common stock											$ 15.3	$ 1.6
Percentage of fixed commission expense of gross proceeds of shares sold in ATM agreement					3.00%
Lincoln Park Capital Fund, LLC ("Investor")
Class of Stock
Value of shares which may be sold				$ 35.0		$ 25.3					$ 25.3		25.3
Duration over which common stock purchase agreement may be sold				36 months
Stock issued (in shares)				269,749							800,000	600,000
Net proceeds from offering of common stock											$ 4.2	$ 1.0
Common Stock [Member]
Class of Stock
Stock issued (in shares)						647,000	2,076,000	3,195,000	3,607,000	854,000
Common Stock [Member] | "At-The-Market" Equity Program
Class of Stock
Net proceeds from offering of common stock					$ 50.0
April 2020 Private Placement
Class of Stock
Stock issued (in shares)		3,039,514
Shares of common stock that may be purchased upon exercise of warrants		3,039,514
Warrant exercised price (per share)		$ 2.28
Gross proceeds from sale of common stock and warrants excluding the proceeds from exercise of the warrants, if any		$ 5.0
Combined purchase price (per share)		$ 1.645
Stock price underlying warrants (per share)		$ 0.125
April 2020 Private Placement Second Closing
Class of Stock
Stock issued (in shares)		2,747,252
Ownership percentage		19.99%
Shares of common stock that may be purchased upon exercise of warrants		1,373,626
Warrant exercised price (per share)		$ 2.71
Gross proceeds from sale of common stock and warrants excluding the proceeds from exercise of the warrants, if any		$ 5.0
Stock price (in dollars per share)		$ 0.01
April 2020 Private Placement and April 2020 Private Placement Second Closing in Aggregate
Class of Stock
Net proceeds from offering of common stock													$ 9.8
July 2020 Private Placement Second Closing
Class of Stock
Proceeds from issuance	$ 5.1
July 2020 Private Placement Second Closing | Pre-funded Warrants
Class of Stock
Shares of common stock that may be purchased upon exercise of warrants	784,615
Stock price (in dollars per share)	$ 0.01
July 2020 Private Placement Second Closing | Warrants
Class of Stock
Shares of common stock that may be purchased upon exercise of warrants	274,615
Stock price (in dollars per share)	$ 9.75