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Fair Value Measurements - Summary of Gain (Loss) Derivative Instruments (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gain (loss) on derivative instruments $ 4,987 $ (1,200)
Marketable Securities, Unrealized Gain (Loss) 4,842 0
Rochester Royalty Obligation    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gain (loss) on derivative instruments 0 (1,200)
Zinc options    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Net derivative gain (loss) $ 100 $ 0