Income and Mining Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax liabilities:
Mineral properties	$ 0	$ 25,691
Inventory	5	847
Royalty and other long-term debt	1,094	0
Foreign subsidiaries - unremitted earnings	99	50
Gross deferred tax liabilities	1,198	26,588
Deferred tax assets:
Net operating loss carryforwards	241,985	219,192
Deferred Tax Assets, Mineral Properties	1,907	0
Property, plant, and equipment	10,841	20,212
Mining Royalty Tax	7,447	6,764
Capital loss carryforwards	17,341	21,956
Asset retirement obligation	38,761	34,134
Unrealized foreign currency loss and other	3,386	9,133
Royalty and other long-term debt	0	6,235
Accrued expenses	16,849	8,899
Tax credit carryforwards	29,809	29,881
Gross deferred tax assets	368,326	356,406
Valuation allowance	(401,304)	(371,277)
Deferred tax assets, net of valuation allowance	(32,978)	(14,871)
Net deferred tax liabilities	$ 34,176	$ 41,459