Fair Value Measurements - Summary of Level 3 Financial Assets and Liabilities (Details) - Silvertip Mine - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the start of the period	$ 25,000	$ 49,276
Revaluation	0	0
Settlements	(25,000)	(25,000)
Accretion	0	724
Balance at the end of the period	$ 0	$ 25,000