Summary of Significant Accounting Policies (Details) - USD ($)	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2016	Oct. 02, 2014
Business Acquisition [Line Items]
Aggregate deposit to be received							$ 20,000,000.0
Production to be sold, percent							50.00%
Price per ounce under agreement							$ 800
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	$ 9,914,000	$ 15,006,000	$ 15,010,000	$ 10,592,000	$ 25,021,000	$ 0