Fair Value Measurements - Summary of Gain (Loss) Derivative Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (loss) on derivative instruments	$ (864)	$ (11,581)	$ 5,202
Rochester Royalty Obligation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (loss) on derivative instruments	(864)	(4,133)	818
Palmarejo gold production royalty
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (loss) on derivative instruments	0	(5,866)	3,101
Silver and gold options
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (loss) on derivative instruments	$ 0	$ (1,582)	$ 1,283