Summary of Significant Accounting Policies (Details) - USD ($)	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Oct. 02, 2014
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	$ 15,006,000	$ 8,018,000	$ 15,010,000	$ 25,000,000.0	$ 25,021,000	$ 0	$ 0