Fair Value Measurements - Summary of Level 3 Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Available-for-sale Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the start of the period	$ 6,227	$ 6,891
Revaluation	193	(172)
Settlements	(1,853)	(2,152)
Asset Sales	0	0
Balance at the end of the period	4,567	4,567
Silvertip Mine
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the start of the period	48,616	47,965
Revaluation	0	0
Settlements	0	0
Liability Sales	329	980
Balance at the end of the period	$ 48,945	$ 48,945