Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jul. 31, 2017	May 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Write-Down of Manquiri Consideration, VAT Refunds				$ 13,100
Write-Down of Manquiri Consideration, Manquiri Notes Receivable				3,600
Write-Down of Manquiri Consideration, NSR				1,900
Ownership in Metalla Royalty & Streaming Ltd.			19.90%		19.90%
Proceeds from Collection of Notes Receivable		$ 15,000	$ 0	15,000	$ 0
Write-Down of Manquiri Consideration		18,599	$ 0	18,599	$ 0
Senior Notes due 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net unamortized debt issuance costs		$ 4,300		$ 4,300		$ 4,900
Stated interest rate								5.875%
Senior Notes due 2024 | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate		5.875%		5.875%
Non-core three royalties and stream
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total consideration							$ 13,000
Convertible debenture							$ 6,700
Debt Securities | Metalla Royalty & Streaming Ltd.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate		5.00%		5.00%
JDS Silver Holdings Ltd.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Purchase Commitment, Milestones				2
Silvertip Mine
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration payment	$ 25,000
Permitting payment, estimated discount rate (percentage)	2.50%
Resource declaration payment, estimated discount rate (percentage)	2.90%