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Income Taxes (Details 2) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Asset (liability)    
Post-employment benefits other than pensions $ 2,787 $ 4,808
Other reserves 4,223 9,333
Deferred compensation 5,054 10,243
Environmental reserves 1,452 2,231
Inventory 4,636 5,876
Pensions 14,307 23,540
Alternative minimum tax credit 0 1,390
Net operating loss and credit carryforwards 6,374 5,607
Research and development tax credit carryforward 2,466 627
Foreign tax credit carryforward 9,481 4,545
Subtotal 50,780 68,200
Valuation allowance (16,246) (3,990)
Total deferred tax assets 34,534 64,210
Depreciation (10,250) (13,064)
Amortization (2,900) (5,073)
Capitalized interest expense (112) (242)
Mine development (3,621) (6,683)
Derivative instruments and hedging activities (817) (13)
Total deferred tax liabilities (17,700) (25,075)
Net deferred tax asset $ 16,834 $ 39,135