Summary of Significant Accounting Policies Summary of Unearned Income (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Oct. 02, 2014
Contract Liabilities
Deferred Revenue Recognized	$ 7,255,000	$ 8,134,000	$ 15,601,000	$ 15,682,000
Palmarejo gold production royalty
Contract Liabilities
Aggregate deposit to be received					$ 22,000,000.0
Price per ounce under agreement					$ 800
Production to be sold, percent					50.00%
Franco-Nevada
Contract Liabilities
Opening Balance	9,030,000	10,505,000	9,376,000	11,061,000
Deferred Revenue Recognized	(255,000)	(116,000)	(601,000)	(672,000)
Closing Balance	8,775,000	10,389,000	8,775,000	10,389,000
Kensington
Contract Liabilities
Opening Balance	7,104,000	8,018,000	15,003,000	15,010,000
Deferred Revenue, Additions	14,900,000	15,006,000	15,001,000	15,006,000
Deferred Revenue Recognized	7,000,000	8,018,000	(15,000,000)	(15,010,000)
Closing Balance	15,004,000	$ 15,006,000	15,004,000	$ 15,006,000
Kensington | December 2020 Prepayment [Member]
Contract Liabilities
Opening Balance	15,000,000.0		15,000,000.0
Deferred Revenue Recognized			15,000,000.0
Closing Balance	$ 15,000,000.0		$ 15,000,000.0