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Consolidated Statements of Stockholders' Equity (Deficit) (Unaudited) (Parentheticals) - shares
3 Months Ended
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Common Stock [Member]            
Employee stock purchase plan sales, shares (in shares) 137,000   110,000 68,000   47,000
Exercise of stock options, shares (in shares)         100,000 160,000
Warrants issued (in shares)   3,511