Summary of Significant Accounting Policies (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Jun. 30, 2019	Dec. 31, 2017	Oct. 02, 2014
Business Acquisition [Line Items]
Impairment of long-lived assets	$ 0	$ 250,814,000	$ 0
Certificates of Deposit, at Carrying Value	9,500,000	8,800,000
Capitalized Exploratory Well Costs that Have Been Capitalized for Period of One Year or Less	8,000,000.0	7,500,000
Palmarejo gold production royalty
Business Acquisition [Line Items]
Aggregate deposit to be received							$ 22,000,000.0
Production to be sold, percent							50.00%
Price per ounce under agreement							$ 800
Kensington
Business Acquisition [Line Items]
Revenue liability	$ 15,003,000	$ 15,009,000	$ 0	$ 15,000,000.0	$ 25,000,000.0	$ 0