Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	6 Months Ended
	Oct. 31, 2017	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jul. 31, 2017	May 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Ownership in Metalla Royalty & Streaming Ltd.				19.90%
Senior Notes due 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net unamortized debt issuance costs		$ 4.5	$ 4.9
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%
Non-core three royalties and stream
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total consideration					$ 13.0
Convertible debenture					$ 6.7
Debt Securities | Metalla Royalty & Streaming Ltd.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate			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	$ 25.0
Permitting payment, estimated discount rate (percentage)	2.50%
Resource declaration payment, estimated discount rate (percentage)	2.90%