Consolidated Statements of Stockholders' Equity (Deficit) (Parentheticals) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Common Stock [Member]
Exercise of stock options, shares (in shares)	510	283
Employee stock purchase plan sales, shares (in shares)	246	115
Employee stock purchase plan sales, shares (in shares)	246	115
Issuance of common stock, shares (in shares)	22,044	43,621
Warrants issued (in shares)	3,511	11,765