Fair Value Measurements - Summary of Level 3 Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the start of the period	$ 6,891	$ 279
Additions		6,677
Revaluation	(274)	(65)
Settlements	(4,400)	0
Accretion	0	0
Balance at the end of the period	2,217	6,891
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Sales		0
Rochester Royalty Obligation
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the start of the period	0	9,287
Additions		0
Revaluation		864
Settlements		(7,819)
Accretion		0
Sales		(2,332)
Balance at the end of the period		0
Silvertip Mine
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the start of the period	47,965	0
Additions		47,705
Revaluation	0	260
Settlements	0	0
Accretion	1,311	0
Sales		0
Balance at the end of the period	$ 49,276	$ 47,965