Income and Mining Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax liabilities:
Mineral properties	$ 143,773	$ 60,199
Unrealized foreign currency loss and other	1,748	0
Inventory	8,258	4,629
Royalty and other long-term debt	0	8,685
Gross deferred tax liabilities	153,779	73,513
Deferred tax assets:
Net operating loss carryforwards	155,512	186,005
Property, plant, and equipment	60,286	60,828
Mining Royalty Tax	11,797	6,359
Capital loss carryforwards	19,881	6,770
Asset retirement obligation	25,309	26,951
Foreign subsidiaries - unremitted earnings	1,842	3,685
Unrealized foreign currency loss and other	218	7,413
Accrued expenses	13,512	15,193
Tax credit carryforwards	45,277	29,227
Gross deferred tax assets	333,634	342,431
Valuation allowance	(282,868)	(338,539)
Deferred tax assets, net of valuation allowance	50,766	3,892
Net deferred tax liabilities	$ 103,013	$ 69,621