Note 10 - Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2020

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
$25	$0.005	$0.02
$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 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
May 7, 2019	$—	$0.0025	$0.0025	$1.2	June 2019
August 5, 2019	$—	$0.0025	$0.0025	$1.2	September 2019
November 6, 2019	$—	$0.0025	$0.0025	$1.3	December 2019
February 21, 2020	$—	$0.0025	$0.0025	$1.3	March 2020
May 6, 2020	$—	$0.0025	$0.0025	$1.3	June 2020
August 5, 2020	$—	$0.0025	$0.0025	$1.3	September 2020
November 6, 2020	$0.005	$0.00375	$0.00875	$4.6	December 2020

Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity [Table Text Block]

	Shares			Weighted Average Grant Date Fair Value per Share
Unvested, January 1, 2020		3,997,168		$	2.46
Granted (unvested)		1,688,111		$	3.03
Canceled		(70,236	)	$	2.08
Distributed (vested)		(1,678,909	)	$	2.83
Unvested, December 31, 2020		3,936,134		$	2.55

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

1,988,079	in June 2021
1,428,145	in June 2022
519,910	in June 2023