Stockholders' Equity (Narrative) (Details) $ / shares in Units, $ in Millions	Dec. 04, 2020 USD ($) $ / shares shares	Jan. 22, 2020 shares	Dec. 24, 2019 USD ($) $ / shares shares	May 07, 2019 USD ($) $ / shares shares	Sep. 30, 2021 vote shares	Dec. 31, 2020 shares	May 03, 2019 closing
Subsidiary, Sale of Stock [Line Items]
Common stock, shares outstanding (in shares)					87,047,832	67,432,712
Common stock voting rights votes | vote					1
Number of years for increase of authorized shares		10 years
Total common shares reserved for future issuance (in shares)					91,616,000
Number of closings | closing							2
Private Placement
Subsidiary, Sale of Stock [Line Items]
Share sold (in shares)	24,341,607			9,730,534
Warrants callable (in shares)	18,256,204			9,730,534
Sale of stock price (in usd per share) | $ / shares	$ 0.7464			$ 1.205
Consideration received | $	$ 19.4			$ 16.7
Shares issued upon conversion (in shares)	10			10
Warrants, term				5 years
Warrant exercise price (in usd per share) | $ / shares				$ 1.08
Milestone Closing
Subsidiary, Sale of Stock [Line Items]
Consideration received | $			$ 24.6
Preferred Class A | Private Placement
Subsidiary, Sale of Stock [Line Items]
Warrants callable (in shares)				4,158,980
Sale of stock price (in usd per share) | $ / shares				$ 10.80
Nonvoting convertible preferred stock issued (in shares)				415,898
Class A-2 Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Reverse stock split ratio			10
Class A-2 Convertible Preferred Stock | Milestone Closing
Subsidiary, Sale of Stock [Line Items]
Share sold (in shares)			3,288,390
Warrants callable (in shares)			32,883,900
Consideration received | $			$ 26.0
Warrants, term			5 years
Warrant exercise price (in usd per share) | $ / shares			$ 0.666
Class A-3 Convertible Preferred Stock | Private Placement
Subsidiary, Sale of Stock [Line Items]
Warrants callable (in shares)	2,047,276
Sale of stock price (in usd per share) | $ / shares	$ 6.22
Nonvoting convertible preferred stock issued (in shares)	272,970
Warrants, term	5 years
Warrant exercise price (in usd per share) | $ / shares	$ 0.7464
Warrants | Preferred Class A | Private Placement
Subsidiary, Sale of Stock [Line Items]
Sale of stock price (in usd per share) | $ / shares				$ 0.125
Warrants | Class A-3 Convertible Preferred Stock | Private Placement
Subsidiary, Sale of Stock [Line Items]
Sale of stock price (in usd per share) | $ / shares	$ 0.125
Certain Directors and Executive Officers | Private Placement
Subsidiary, Sale of Stock [Line Items]
Share sold (in shares)	833,208			526,083
Certain Directors and Executive Officers | Preferred Class A | Private Placement
Subsidiary, Sale of Stock [Line Items]
Warrants callable (in shares)				526,083
Proceeds from issuance of right to purchase common shares | $				$ 0.6
Certain Directors and Executive Officers | Class A-2 Convertible Preferred Stock | Milestone Closing
Subsidiary, Sale of Stock [Line Items]
Share sold (in shares)			121,581
Warrants callable (in shares)			1,215,810
Consideration received | $			$ 1.0
Certain Directors and Executive Officers | Class A-3 Convertible Preferred Stock | Private Placement
Subsidiary, Sale of Stock [Line Items]
Warrants callable (in shares)	624,906
Proceeds from issuance of right to purchase common shares | $	$ 0.6
2019 Plan
Subsidiary, Sale of Stock [Line Items]
Number of additional shares authorized (in shares)		4,166,860
Number of additional shares authorized yearly increase		5.00%
Common stock available for future grant (in shares)					853,111
Total common shares reserved for future issuance (in shares)					10,172,886