Note 9 - Stockholders' Equity - Common Stock Dividends Declared by Our Board of Directors (Details) $ / shares in Units, $ in Millions	Dec. 31, 2016 USD ($) $ / shares
Declared May 5, 2014 [Member]
Silver-price-linked component per share (in dollars per share)
Minimum annual component per share (in dollars per share)	0.0025
Special dividend per share (in dollars per share)
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 0.9
Declared July 31, 2014 [Member]
Silver-price-linked component per share (in dollars per share)
Minimum annual component per share (in dollars per share)	0.0025
Special dividend per share (in dollars per share)
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 0.9
Declared November 5, 2014 [Member]
Silver-price-linked component per share (in dollars per share)
Minimum annual component per share (in dollars per share)	0.0025
Special dividend per share (in dollars per share)
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 0.9
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
Special dividend per share (in dollars per share)
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
Special dividend per share (in dollars per share)
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
Special dividend per share (in dollars per share)
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
Special dividend per share (in dollars per share)
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
Special dividend per share (in dollars per share)
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
Special dividend per share (in dollars per share)
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
Special dividend per share (in dollars per share)
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
Special dividend per share (in dollars per share)
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
Special dividend per share (in dollars per share)
Total dividend per share (in dollars per share)	$ 0.0025
Total dividend amount (in Dollars) | $	$ 1.0