Note 10 - Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2018

Notes Tables

Schedule of Potential Per Share Dividend Amounts Quarterly Price Levels [Table Text Block]

Quarterly average realized silver price per ounce	Quarterly dividend per share	Annual dividend per share
$30	$0.01	$0.04
$35	$0.02	$0.08
$40	$0.03	$0.12
$45	$0.04	$0.16
$50	$0.05	$0.20
$55	$0.06	$0.24
$60	$0.07	$0.28

Schedule of Dividends Payable [Table Text Block]

	(A)	(B)	(A+B)
Declaration date	Silver-price- linked component per share	Minimum annual component per share	Total dividend per share	Total dividend amount (in millions)	Month of payment
February 20, 2016	$—	$0.0025	$0.0025	$0.9	March 2016
May 4, 2016	$—	$0.0025	$0.0025	$1.0	June 2016
August 3, 2016	$—	$0.0025	$0.0025	$1.0	September 2016
November 4, 2016	$—	$0.0025	$0.0025	$1.0	December 2016
February 21, 2017	$—	$0.0025	$0.0025	$1.0	March 2017
May 4, 2017	$—	$0.0025	$0.0025	$1.0	June 2017
August 3, 2017	$—	$0.0025	$0.0025	$1.0	September 2017
November 7, 2017	$—	$0.0025	$0.0025	$1.0	December 2017
February 14, 2018	$—	$0.0025	$0.0025	$1.0	March 2018
May 9, 2018	$—	$0.0025	$0.0025	$1.0	June 2018
August 8, 2018	$—	$0.0025	$0.0025	$1.2	September 2018
November 6, 2018	$—	$0.0025	$0.0025	$1.2	December 2018
February 20, 2019	$—	$0.0025	$0.0025	$1.2	March 2019

Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block]

	Shares			Weighted Average Grant Date Fair Value per Share
Unvested, January 1, 2018		2,095,318		$	4.52
Granted (unvested)		1,975,662		$	3.53
Canceled		(196,594	)	$	3.48
Distributed (vested)		(1,184,918	)	$	3.91
Unvested, December 31, 2018		2,689,468		$	4.14

Schedule of Share-based Compensation, Unvested Shares Expected to Vest [Table Text Block]

1,246,291	in June 2019
866,680	in June 2020
576,497	in June 2021