Summary of Significant Accounting Policies Summary of Unearned Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Contract Liabilities
Revenue Recognized	$ (5,485)	$ (15,250)		$ (21,167)	$ (16,008)
Franco-Nevada
Contract Liabilities
Opening Balance	10,389	12,160	$ 11,061	11,061	12,918	$ 12,918
Revenue Recognized	393	545		1,065	1,303
Closing Balance	9,996	11,615	10,389	9,996	11,615	11,061
Kensington
Contract Liabilities
Opening Balance	15,006	25,021	15,010	15,010	0	0
Revenue Recognized	(5,200)	(14,705)	(15,000)	(20,210)	(14,705)	(25,000)
Closing Balance	9,914	10,592	15,006	9,914	10,592	$ 15,010
Deferred Revenue, Additions	108	$ 276		$ 15,114	$ 25,297
Kensington | June 2020 Prepayment [Member]
Contract Liabilities
Opening Balance	$ 15,000
Closing Balance			$ 15,000