Summary of Significant Accounting Policies Summary of Unearned Income (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Oct. 02, 2014
Contract Liabilities
Deferred Revenue Recognized	$ 8,346,000	$ 7,548,000
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	9,376,000	11,061,000
Deferred Revenue Recognized	(346,000)	(556,000)
Closing Balance	9,030,000	10,505,000
Kensington
Contract Liabilities
Opening Balance	15,003,000	15,010,000
Deferred Revenue, Additions	101,000	132,000
Deferred Revenue Recognized	(8,000,000)	(7,124,000)
Closing Balance	7,104,000	$ 8,018,000
Kensington | December 2020 Prepayment [Member]
Contract Liabilities
Opening Balance	15,000,000.0
Deferred Revenue Recognized	8,000,000.0
Closing Balance	$ 7,100,000