Note 10 - Stockholders' Equity - Common Stock Dividends Declared by Our Board of Directors (Details) $ / shares in Units, $ in Millions	Dec. 31, 2018 USD ($) $ / shares
Declared February 20, 2016 [Member]
Silver-price-linked component per share (in dollars per share)
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 February 17, 2015 [Member]
Total dividend amount (in Dollars) | $	$ 0.9
Declared May 4, 2016 [Member]
Silver-price-linked component per share (in dollars per share)
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 6, 2015 [Member]
Total dividend amount (in Dollars) | $	$ 1.0
Declared August 3, 2016 [Member]
Silver-price-linked component per share (in dollars per share)
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 6, 2015 [Member]
Total dividend amount (in Dollars) | $	$ 1.0
Declared November 4, 2016 [Member]
Silver-price-linked component per share (in dollars per share)
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 November 3, 2015 [Member]
Total dividend amount (in Dollars) | $	$ 1.0
Declared February 21, 2017 [Member]
Silver-price-linked component per share (in dollars per share)
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 November 7, 2017 [Member]
Silver-price-linked component per share (in dollars per share)
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 14, 2018 [Member]
Silver-price-linked component per share (in dollars per share)
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 9,2018 [Member]
Silver-price-linked component per share (in dollars per share)
Minimum annual component per share (in dollars per share)	0.0025
Total dividend per share (in dollars per share)	$ 0.0025
Declared May 4, 2017 [Member]
Total dividend amount (in Dollars) | $	$ 1.0
Declared August 8, 2018 [Member]
Silver-price-linked component per share (in dollars per share)
Minimum annual component per share (in dollars per share)	0.0025
Total dividend per share (in dollars per share)	$ 0.0025
Declared August 3, 2017 [Member]
Total dividend amount (in Dollars) | $	$ 1.2
Declared November 6, 2018 [Member]
Silver-price-linked component per share (in dollars per share)
Minimum annual component per share (in dollars per share)	0.0025
Total dividend per share (in dollars per share)	0.0025
Declared February 20, 2019 [Member]
Silver-price-linked component per share (in dollars per share)
Minimum annual component per share (in dollars per share)	0.0025
Total dividend per share (in dollars per share)	$ 0.0025