Fair Value Measurements - Summary of Level 3 Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
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	$ 6,891
Revaluation		(274)
Settlements		(4,400)
Accretion		0
Balance at the end of the period		2,217
Silvertip Mine
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the start of the period	49,276	47,965
Revaluation	0	0
Settlements	(25,000)	0
Accretion	724	1,311
Balance at the end of the period	$ 25,000	$ 49,276