Note 7 - Stockholders' Equity	12 Months Ended
Dec. 31, 2020
Notes to Financial Statements
Stockholders' Equity Note Disclosure [Text Block]	Note 7 — Stockholders’ Equity Sales of Common Stock On November 8, 2017, the Company filed a $200,000 shelf registration statement on Form S-3 with the SEC. This shelf registration statement became effective on December 12, 2017 and allowed the Company to sell securities, including common stock, preferred stock, depository shares, stock purchase contracts, warrants and units, from time to time at prices and on terms to be determined at the time of sale. On August 6, 2018, the Company completed an underwritten public offering of 10,455 shares of its common stock, offered at a price to the public of $5.50 per share, including shares issued pursuant to the underwriters’ 30-day option to purchase additional shares, which was exercised in full. The net proceeds from this offering were approximately $53,400 after deducting underwriting discounts and commissions and estimated offering expenses. On November 18, 2019, the Company completed an underwritten public offering of 43,621 shares of its common stock, offered at a price to the public of $1.45 per share, including shares issued pursuant to the underwriters’ 30-day option to purchase additional shares, which was exercised in full. The net proceeds from this offering were approximately $58,500 after deducting underwriting discounts and commissions and estimated offering expenses. On November 21, 2019, the Company completed an offering of pre-funded warrants to purchase up to 11,765 shares of its common stock at a price of $1.69 per warrant. Each pre-funded warrant is exercisable subject to conditions in the warrant agreement into 1 share of common stock at an exercise price of $0.01 per share. The net proceeds from this offering were $19,882, excluding any proceeds the Company may receive upon the subsequent exercise of the pre-funded warrants. All warrants issued in this offering remain outstanding at December 31, 2020. On April 24, 2020, the Company filed a $500,000 shelf registration statement on Form S-3 with the SEC. This shelf registration statement became effective on May 14, 2020 and allows the Company to sell securities, including common stock, preferred stock, depository shares, purchase contracts, warrants, debt securities, and units, from time to time at prices and on terms to be determined at the time of sale. On June 1, 2020, the Company issued 22,044 shares of common stock to the public at a purchase price of $4.50 per share and pre-funded warrants to purchase 3,511 shares of common stock at a purchase of $4.49 per pre-funded warrant, for total net proceeds to the Company of $108,096 million after deducting underwriting discounts and commissions and other offering expenses. Each pre-funded warrant is exercisable subject to conditions in the warrant agreement into 1 share of common stock at an exercise price of $0.01 per share. All warrants issued in this offering remain outstanding at December 31, 2020.

Note 7 - Stockholders' Equity

12 Months Ended

Dec. 31, 2020

Stockholders' Equity Note Disclosure [Text Block]

Note 7 — Stockholders’ Equity

Sales of Common Stock

On November 8, 2017, the Company filed a $200,000 shelf registration statement on Form S-3 with the SEC. This shelf registration statement became effective on December 12, 2017 and allowed the Company to sell securities, including common stock, preferred stock, depository shares, stock purchase contracts, warrants and units, from time to time at prices and on terms to be determined at the time of sale.

On August 6, 2018, the Company completed an underwritten public offering of 10,455 shares of its common stock, offered at a price to the public of $5.50 per share, including shares issued pursuant to the underwriters’ 30-day option to purchase additional shares, which was exercised in full. The net proceeds from this offering were approximately $53,400 after deducting underwriting discounts and commissions and estimated offering expenses.

On November 18, 2019, the Company completed an underwritten public offering of 43,621 shares of its common stock, offered at a price to the public of $1.45 per share, including shares issued pursuant to the underwriters’ 30-day option to purchase additional shares, which was exercised in full. The net proceeds from this offering were approximately $58,500 after deducting underwriting discounts and commissions and estimated offering expenses.

On November 21, 2019, the Company completed an offering of pre-funded warrants to purchase up to 11,765 shares of its common stock at a price of $1.69 per warrant. Each pre-funded warrant is exercisable subject to conditions in the warrant agreement into 1 share of common stock at an exercise price of $0.01 per share. The net proceeds from this offering were $19,882, excluding any proceeds the Company may receive upon the subsequent exercise of the pre-funded warrants. All warrants issued in this offering remain outstanding at December 31, 2020.

On April 24, 2020, the Company filed a $500,000 shelf registration statement on Form S-3 with the SEC. This shelf registration statement became effective on May 14, 2020 and allows the Company to sell securities, including common stock, preferred stock, depository shares, purchase contracts, warrants, debt securities, and units, from time to time at prices and on terms to be determined at the time of sale.

On June 1, 2020, the Company issued 22,044 shares of common stock to the public at a purchase price of $4.50 per share and pre-funded warrants to purchase 3,511 shares of common stock at a purchase of $4.49 per pre-funded warrant, for total net proceeds to the Company of $108,096 million after deducting underwriting discounts and commissions and other offering expenses. Each pre-funded warrant is exercisable subject to conditions in the warrant agreement into 1 share of common stock at an exercise price of $0.01 per share. All warrants issued in this offering remain outstanding at December 31, 2020.