Summary of Significant Accounting Policies Summary of Unearned Income (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Contract Liabilities
Revenue Recognized			$ (16,702)	$ (1,857)	$ 0
Franco-Nevada
Contract Liabilities
Opening Balance		$ 11,061	11,061	12,918	14,883
Revenue Recognized			1,685	1,857	1,965
Closing Balance	$ 9,376		9,376	11,061	12,918
Kensington
Contract Liabilities
Opening Balance	15,000	15,009	15,009	0	0
Revenue Recognized	(15,000)	$ (15,000)	(30,183)	(25,000)	0
Closing Balance	15,003		15,003	15,009	0
Deferred Revenue, Additions			30,177	$ 40,009	$ 0
Kensington | June 2020 Prepayment [Member]
Contract Liabilities
Opening Balance	15,000
Kensington | December 2020 Prepayment [Member]
Contract Liabilities
Opening Balance	15,000
Closing Balance	$ 15,000		$ 15,000