Fair Value Measurements - Summary of Level 3 Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Equity and debt securities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the start of the period		$ 2,217
Revaluation		59
Settlements		(2,276)
Accretion		0
Balance at the end of the period	$ 0	0
Silvertip Mine
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the start of the period	49,455	49,276
Revaluation	0	0
Settlements	0	0
Accretion	180	359
Balance at the end of the period	$ 49,635	$ 49,635