Common Stock Issuance (Narrative) (Details) - USD ($)	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Cash received from stock sale	$ 61,464,000	$ 1,481,000
December 2019 Agreement
Number of shares of common stock issued	22,200,000
Cash received from stock sale	$ 15,600,000
Terms and provisions	In December 2019, we entered into a Common Stock Purchase Agreement with Lincoln Park granting us the right to sell shares of our common stock having an aggregate value of up to $16.0 million. Under the terms of the agreement, Lincoln Park made an initial purchase of 1.5 million shares of common stock for $1.0 million at a purchase price of $0.6531 per share. Subject to various limitations and conditions set forth in the agreement, we were able to sell up to an additional $15.0 million in shares of common stock, from time to time, at our sole discretion to Lincoln Park over a 24-month period beginning December 2019. In consideration for entering into the agreement, we issued 375,000 shares of our common stock, having a value of $277,000, based on the closing stock price at the date of grant, to Lincoln Park as a commitment fee. We incurred an additional $90,000 in issuance costs. As of December 31, 2020, we had completed sales under such sales agreement, having sold 22.2 million shares for net proceeds of $15.6 million.
November 2020
Number of shares of common stock issued	4,900,000
Cash received from stock sale	$ 9,600,000
Terms and provisions	In November 2020, we entered into a $10.0 million ATM equity offering agreement with Craig-Hallum Capital Group. Under the agreement we were able, from time to time, at our discretion to offer and sell shares of our common stock having an aggregate value of up to $10.0 million through Craig-Hallum. As of December 31, 2020, we had completed sales under such sales agreement, having sold 4.9 million shares for net proceeds of $9.6 million.
December 2020
Number of shares of common stock issued	2,100,000
Cash received from stock sale	$ 12,700,000
Terms and provisions	In December 2020, we entered into a $13.0 million ATM equity offering agreement with Craig-Hallum. Under the agreement we were able, from time to time, at our discretion to offer and sell shares of our common stock having an aggregate value of up to $13.0 million through Craig-Hallum. As of December 31, 2020, we had issued 1.0 million shares for net proceeds of $6.1 million that was received in January 2021. The $6.1 million was classified as subscriptions receivable on our December 31, 2020 balance sheet and is not included in the cash balance as of December 31, 2020. In January 2021, we issued 1.1 million shares of our common stock for net proceeds of $6.6 million under the agreement. In total, we have issued 2.1 million shares of our common stock for net proceeds of $12.7 million under this ATM agreement. No further shares are available for sales under this agreement.
February 2021
Number of shares of common stock issued	2,500,000
Cash received from stock sale	$ 48,800,000
Terms and provisions	In February 2021, we entered into a $50.0 million ATM equity offering agreement with Craig-Hallum. Under the agreement we were able, at our discretion, to offer and sell shares of our common stock having an aggregate value of up to $50.0 million through Craig-Hallum. We have issued 2.5 million shares of our common stock for net proceeds of $48.8 million under this ATM agreement. No further shares are available for sales under this agreement.

$ 61,464,000

$ 1,481,000

22,200,000

In December 2019, we entered into a Common Stock Purchase Agreement with Lincoln Park granting us the right to sell shares of our common stock having an aggregate value of up to $16.0 million. Under the terms of the agreement, Lincoln Park made an initial purchase of 1.5 million shares of common stock for $1.0 million at a purchase price of $0.6531 per share. Subject to various limitations and conditions set forth in the agreement, we were able to sell up to an additional $15.0 million in shares of common stock, from time to time, at our sole discretion to Lincoln Park over a 24-month period beginning December 2019. In consideration for entering into the agreement, we issued 375,000 shares of our common stock, having a value of $277,000, based on the closing stock price at the date of grant, to Lincoln Park as a commitment fee. We incurred an additional $90,000 in issuance costs. As of December 31, 2020, we had completed sales under such sales agreement, having sold 22.2 million shares for net proceeds of $15.6 million.

Number of shares of common stock issued

4,900,000

Terms and provisions

In November 2020, we entered into a $10.0 million ATM equity offering agreement with Craig-Hallum Capital Group. Under the agreement we were able, from time to time, at our discretion to offer and sell shares of our common stock having an aggregate value of up to $10.0 million through Craig-Hallum. As of December 31, 2020, we had completed sales under such sales agreement, having sold 4.9 million shares for net proceeds of $9.6 million.

Number of shares of common stock issued

2,100,000

Terms and provisions

In December 2020, we entered into a $13.0 million ATM equity offering agreement with Craig-Hallum. Under the agreement we were able, from time to time, at our discretion to offer and sell shares of our common stock having an aggregate value of up to $13.0 million through Craig-Hallum. As of December 31, 2020, we had issued 1.0 million shares for net proceeds of $6.1 million that was received in January 2021. The $6.1 million was classified as subscriptions receivable on our December 31, 2020 balance sheet and is not included in the cash balance as of December 31, 2020. In January 2021, we issued 1.1 million shares of our common stock for net proceeds of $6.6 million under the agreement. In total, we have issued 2.1 million shares of our common stock for net proceeds of $12.7 million under this ATM agreement. No further shares are available for sales under this agreement.

Number of shares of common stock issued

2,500,000

Terms and provisions

In February 2021, we entered into a $50.0 million ATM equity offering agreement with Craig-Hallum. Under the agreement we were able, at our discretion, to offer and sell shares of our common stock having an aggregate value of up to $50.0 million through Craig-Hallum. We have issued 2.5 million shares of our common stock for net proceeds of $48.8 million under this ATM agreement. No further shares are available for sales under this agreement.