Consolidated Statements of Stockholders' Equity (Parentheticals) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Common Stock [Member]
Exercise of stock options, shares (in shares)	510,000	283,000
Employee stock purchase plan sales, shares (in shares)	246,000	115,000	92,000
Issuance of common stock, shares (in shares)	22,044,000	43,621,000	10,455,000
Warrants issued (in shares)	3,511,000	11,765,000