Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended	12 Months Ended
	Oct. 31, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2015	Sep. 30, 2017	Jul. 31, 2017	May 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain on repurchase of the Rochester royalty obligation		$ 0	$ 2,332	$ 0
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			$ 4,900
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%
Senior Notes due 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net unamortized debt issuance costs								$ 2,000
Senior Notes due 2021 | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate			7.875%
Rochester Royalty Obligation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfer of financial assets accounted for as sales, amount							$ 5,000
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%
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%