Inventories, net - Schedule of Mine site period costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Inventory Disclosure [Abstract]
Production related costs	$ 2,264	$ 11,996	$ 12,384	$ 18,631
Capitalized depreciation and amortization	170	874	594	1,431
Total	$ 2,434	$ 12,870	$ 12,978	$ 20,062