Fair Value Measurements - Summary of Level 3 Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the start of the period	$ 279	$ 10
Asset Purchases	6,677	0
Revaluation	(65)	272
Settlements	0	3
Asset Sales	0	0
Balance at the end of the period	6,891	279
Palmarejo royalty obligation
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the start of the period	0	4,957
Liability Purchases		0
Revaluation		5,866
Settlements		(10,823)
Liability Sales		0
Balance at the end of the period		0
Rochester Royalty Obligation
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the start of the period	9,287	9,593
Liability Purchases	0	0
Revaluation	864	4,133
Settlements	(7,819)	(4,439)
Liability Sales	(2,332)	0
Balance at the end of the period	0	9,287
Silvertip Mine
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the start of the period	0
Liability Purchases	47,705
Revaluation	260
Settlements	0
Liability Sales	0
Balance at the end of the period	$ 47,965	$ 0