Stockholders' Equity (Deficit) - Common Stock Warrants (Details) - $ / shares	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Class of Warrant or Right
Number of Shares	8,732,230	16,967,828
Weighted-Average Exercise Price	$ 2.04	$ 1.28
Warrants
Class of Warrant or Right
Weighted-Average Exercise Price			$ 1.52
Preferred stock
Class of Warrant or Right
Weighted-Average Exercise Price		$ 152
Liability Classified Warrants
Class of Warrant or Right
Number of Shares		2,368,400
December 2019 Series B1 warrants
Class of Warrant or Right
Number of Shares		2,368,400
Weighted-Average Exercise Price	$ 1.52	$ 1.52
Equity Classified Warrants
Class of Warrant or Right
Number of Shares	8,732,230	14,599,428
May 2013 Warrants
Class of Warrant or Right
Number of Shares	15,437	1,949,754
Weighted-Average Exercise Price	$ 0.08	$ 0.08
September 2013 Warrants
Class of Warrant or Right
Number of Shares	4,096	514,756
Weighted-Average Exercise Price	$ 0.08	$ 0.08
February 2014 Warrants
Class of Warrant or Right
Number of Shares	2,171	266,006
Weighted-Average Exercise Price	$ 0.08	$ 0.08
April 2020 Private Placement first closing warrants
Class of Warrant or Right
Number of Shares	3,039,514	3,039,514
Weighted-Average Exercise Price	$ 2.28
April 2020 Private Placement second closing warrants
Class of Warrant or Right
Number of Shares	1,373,626	1,373,626
Weighted-Average Exercise Price	$ 2.71
April 2020 Private Placement second closing warrants
Class of Warrant or Right
Number of Shares	1,143,428	2,677,311
Weighted-Average Exercise Price	$ 0.01
July 2020 Private Placement first closing warrants
Class of Warrant or Right
Number of Shares	389,731	2,014,234
Weighted-Average Exercise Price	$ 0.01
July 2020 Private Placement first closing warrants
Class of Warrant or Right
Number of Shares	2,764,227	2,764,227
Weighted-Average Exercise Price	$ 2.58