Consolidated Statements of Stockholders' Equity - USD ($)	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Gain (Loss)	Treasury Stock
Balance, shares at Dec. 31, 2019		106,679,000
Balance, value at Dec. 31, 2019	$ 117,101,000	$ 106,000	$ 1,462,172,000	$ (1,341,444,000)	$ 84,000	$ (3,817,000)
Stock-based compensation	13,274,000	0	13,274,000	0	0	0
Stock Issued During Period, Value, New Issues	6,963,000	$ 4,000	6,959,000	0	0	0
Stock Issued During Period, Shares, New Issues		3,709,000
Issuance of Stock and Warrants for Services or Claims	0
Sale of Stock, Consideration Received Per Transaction	63,007,000	$ 20,000	62,987,000	0	0	0
Common stock, shares issued		10,369,000
Issuance of common stock under Equity Incentive Plans, shares		1,219,000
Issuance of common stock under Equity Incentive Plans, value	2,000	$ 2,000	0	0	0	0
Repurchase of common stock	(1,026,000)	0	0	0	0	(1,026,000)
Net loss	(58,574,000)	0	0	(58,574,000)	0	0
Unrealized gain (loss) on investments	(90,000)	$ 0	0	0	(90,000)	0
Balance, shares at Dec. 31, 2020		142,289,000
Balance, value at Dec. 31, 2020	$ 156,371,000	$ 142,000	1,561,096,000	(1,400,018,000)	(6,000)	(4,843,000)
Sale of Stock, Number of Shares Issued in Transaction	20,312,500	20,313,000
Conversion of Stock, Amount Issued	$ 15,714,000	$ 10,000	15,704,000	0	0	0
Share price in November 20 ATM Offering	1.992
Proceeds from Nov 2020 ATM Offering	7,000,000
Commissions from December 2020 Common Stock Offering	1,800,000
Offering expenses from December 20 common stock offering	200,000
Issuance of Treasury Stock	$ 0
Shares Issued, Price Per Share	$ 3.20
Proceeds from December 2020 common stock offering	$ 63,000,000
Stock-based compensation	10,577,000	0	10,577,000	0	0	0
Stock Issued During Period, Value, New Issues	35,491,000	$ 6,000	35,485,000	0	0	0
Stock Issued During Period, Shares, New Issues		6,177,000
Issuance of Stock and Warrants for Services or Claims	0
Issuance of common stock under Equity Incentive Plans, shares		1,616,000
Issuance of common stock under Equity Incentive Plans, value	1,593,000	$ 2,000	1,591,000	0	0	0
Repurchase of common stock	(2,675,000)	0	0	0	0	(2,675,000)
Net loss	(87,758,000)	0	0	(87,758,000)	0	0
Unrealized gain (loss) on investments	(4,000)	$ 0	0	0	(4,000)	0
Balance, shares at Dec. 31, 2021		150,082,000
Balance, value at Dec. 31, 2021	$ 113,595,000	$ 150,000	1,608,749,000	(1,487,776,000)	(10,000)	(7,518,000)
Shares sold in January 2021 ATM offering	2,000,000
Proceeds from January 2021 ATM Offering	$ 16,400,000
Issuance of Treasury Stock	0
Stock-based compensation	11,520,000	0	11,520,000	0	0	0
Stock Issued During Period, Value, New Issues	94,205,000	$ 39,000	94,166,000	0	0	0
Stock Issued During Period, Shares, New Issues		39,100,000
Issuance of Stock and Warrants for Services or Claims	1,030,000	$ 0	1,030,000	0	0	0
Issuance of common stock under Equity Incentive Plans, shares		32,000
Issuance of common stock under Equity Incentive Plans, value	0	$ 0	0	0	0	0
Repurchase of common stock	(864,000)	0	0	0	0	(864,000)
Net loss	(101,944,000)	0	0	(101,944,000)	0	0
Unrealized gain (loss) on investments	(418,000)	$ 0	0	0	(418,000)	0
Balance, shares at Dec. 31, 2022		189,214,000
Balance, value at Dec. 31, 2022	117,124,000	$ 189,000	1,709,144,000	(1,589,720,000)	(428,000)	(2,061,000)
Stock Issued During Period, Shares, Treasury Stock Reissued		0
Issuance of Treasury Stock	$ 0	$ 0	$ (6,321,000)	$ 0	$ 0	$ (6,321,000)