Note 10 - Stockholders' Equity - Common Stock Dividends Declared by Our Board of Directors (Details) $ / shares in Units, $ in Millions	Dec. 31, 2020 USD ($) $ / shares
Declared February 14, 2018 [Member]
Minimum annual component per share (in dollars per share)	$ 0.0025
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 1.0
Declared May 9,2018 [Member]
Minimum annual component per share (in dollars per share)	$ 0.0025
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 1.0
Declared August 8, 2018 [Member]
Minimum annual component per share (in dollars per share)	$ 0.0025
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 1.2
Declared November 6, 2018 [Member]
Minimum annual component per share (in dollars per share)	$ 0.0025
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 1.2
Declared February 20, 2019 [Member]
Minimum annual component per share (in dollars per share)	$ 0.0025
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 1.2
Declared May 7, 2019 [Member]
Minimum annual component per share (in dollars per share)	$ 0.0025
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 1.2
Declared August 5, 2019 [Member]
Minimum annual component per share (in dollars per share)	$ 0.0025
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 1.2
Declared November 6, 2019 [Member]
Minimum annual component per share (in dollars per share)	$ 0.0025
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 1.3
Declared February 21, 2020 [Member]
Minimum annual component per share (in dollars per share)	$ 0.0025
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 1.3
Declared May 6, 2020 [Member]
Minimum annual component per share (in dollars per share)	$ 0.0025
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 1.3
Declared August 5, 2020 [Member]
Minimum annual component per share (in dollars per share)	$ 0.0025
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 1.3
Declared November 6, 2020 [Member]
Minimum annual component per share (in dollars per share)	$ 0.00375
Total dividend per share (in dollars per share)	$ 0.00875
Total dividend amount (in Dollars) | $	$ 4.6
Silver-price-linked component per share (in dollars per share)	$ 0.005