Income and Mining Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax liabilities:
Mineral properties	$ 25,691	$ 118,852
Inventory	847	4,513
Foreign subsidiaries - unremitted earnings	50	7
Gross deferred tax liabilities	26,588	123,372
Deferred tax assets:
Net operating loss carryforwards	219,192	189,840
Property, plant, and equipment	20,212	7,779
Mining Royalty Tax	6,764	8,980
Capital loss carryforwards	21,956	23,003
Asset retirement obligation	34,134	27,980
Unrealized foreign currency loss and other	9,133	8,387
Royalty and other long-term debt	6,235	3,821
Accrued expenses	8,899	14,247
Tax credit carryforwards	29,881	33,897
Gross deferred tax assets	356,406	317,934
Valuation allowance	(371,277)	(272,839)
Deferred tax assets, net of valuation allowance	(14,871)	45,095
Net deferred tax liabilities	$ 41,459	$ 78,277