Income and Mining Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax liabilities:
Mineral properties	$ 118,852	$ 102,573
Unrealized foreign currency loss and other	0	1,748
Inventory	4,513	8,258
Foreign subsidiaries - unremitted earnings	7	0
Gross deferred tax liabilities	123,372	112,579
Deferred tax assets:
Net operating loss carryforwards	189,840	155,512
Property, plant, and equipment	7,779	19,086
Mining Royalty Tax	8,980	11,797
Capital loss carryforwards	23,003	19,881
Asset retirement obligation	27,980	25,309
Foreign subsidiaries - unremitted earnings	0	1,842
Unrealized foreign currency loss and other	8,387	218
Royalty and other long-term debt	3,821	0
Accrued expenses	14,247	13,512
Tax credit carryforwards	33,897	45,277
Gross deferred tax assets	317,934	292,434
Valuation allowance	(272,839)	(282,868)
Deferred tax assets, net of valuation allowance	45,095	9,566
Net deferred tax liabilities	$ 78,277	$ 103,013