Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Oct. 31, 2017	Mar. 31, 2019	Dec. 31, 2018	Jul. 31, 2017
Senior Notes due 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net unamortized debt issuance costs		$ 4.0	$ 4.1
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
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%