Note 9 - Stockholders' Equity - Common Stock Dividends Declared by Our Board of Directors (Details) $ / shares in Units, $ in Millions	Dec. 31, 2017 USD ($) $ / shares
Declared February 17, 2015 [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) | $	$ 0.9
Declared May 6, 2015 [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) | $	$ 0.9
Declared August 6, 2015 [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) | $	$ 0.9
Declared November 3, 2015 [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) | $	$ 0.9
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) | $	$ 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 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 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 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 May 4, 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 August 3, 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) | $	$ 1.0
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) | $	$ 1.0