Note 9 - Shareholders' Equity (Details) - Common Stock Dividends Declared by Our Board of Directors (USD $) In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014
Declared November 8, 2011 [Member]
Dividends Payable [Line Items]
Silver-price-linked component per share	$ 0.02
Total dividend per share	$ 0.02
Total dividend amount (in Dollars)	$ 5.6
Declared February 17, 2012 [Member]
Dividends Payable [Line Items]
Silver-price-linked component per share	$ 0.01
Minimum annual component per share	$ 0.0025
Total dividend per share	$ 0.0125
Total dividend amount (in Dollars)	3.6
Declared May 8, 2012 [Member]
Dividends Payable [Line Items]
Silver-price-linked component per share	$ 0.02
Minimum annual component per share	$ 0.0025
Total dividend per share	$ 0.0225
Total dividend amount (in Dollars)	6.4
Declared August 7, 2012 [Member]
Dividends Payable [Line Items]
Minimum annual component per share	$ 0.0025
Total dividend per share	$ 0.0025
Total dividend amount (in Dollars)	0.7
Declared November 2, 2012 [Member]
Dividends Payable [Line Items]
Silver-price-linked component per share	$ 0.02
Minimum annual component per share	$ 0.0025
Total dividend per share	$ 0.0225
Total dividend amount (in Dollars)	6.4
Declared February 25, 2013 [Member]
Dividends Payable [Line Items]
Minimum annual component per share	$ 0.0025
Special dividend per share	$ 0.01
Total dividend per share	$ 0.0125
Total dividend amount (in Dollars)	3.6
Declared May 10, 2013 [Member]
Dividends Payable [Line Items]
Minimum annual component per share	$ 0.0025
Total dividend per share	$ 0.0025
Total dividend amount (in Dollars)	0.7
Declared August 8, 2013 [Member]
Dividends Payable [Line Items]
Minimum annual component per share	$ 0.0025
Total dividend per share	$ 0.0025
Total dividend amount (in Dollars)	0.9
Declared November 5, 2013 [Member]
Dividends Payable [Line Items]
Minimum annual component per share	$ 0.0025
Total dividend per share	$ 0.0025
Total dividend amount (in Dollars)	0.9
Declared February 21, 2014 [Member]
Dividends Payable [Line Items]
Minimum annual component per share	$ 0.0025
Total dividend per share	$ 0.0025
Total dividend amount (in Dollars)	0.9
Declared May 5, 2014 [Member]
Dividends Payable [Line Items]
Minimum annual component per share	$ 0.0025
Total dividend per share	$ 0.0025
Total dividend amount (in Dollars)	0.9
Declared July 31, 2014 [Member]
Dividends Payable [Line Items]
Minimum annual component per share	$ 0.0025
Total dividend per share	$ 0.0025
Total dividend amount (in Dollars)	0.9
Declared November 5, 2014 [Member]
Dividends Payable [Line Items]
Minimum annual component per share	$ 0.0025
Total dividend per share	$ 0.0025
Total dividend amount (in Dollars)	0.9
Declared February 17, 2015 [Member]
Dividends Payable [Line Items]
Minimum annual component per share	$ 0.0025
Total dividend per share	$ 0.0025
Total dividend amount (in Dollars)	$ 0.9