Income Taxes (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Asset (liability)
Post-employment benefits other than pensions	$ 5,543	$ 10,989
Other reserves	17,764	13,744
Environmental reserves	1,808	1,789
Inventory	4,808us-gaap_DeferredTaxAssetsInventory	3,934us-gaap_DeferredTaxAssetsInventory
Pensions	24,158	9,867
Net operating loss and credit carryforwards	5,714	5,588
Miscellaneous	345us-gaap_DeferredTaxAssetsOther	303us-gaap_DeferredTaxAssetsOther
Subtotal	60,140us-gaap_DeferredTaxAssetsGross	46,214us-gaap_DeferredTaxAssetsGross
Valuation allowance	(4,444)	(4,782)
Total deferred tax assets	55,696us-gaap_DeferredTaxAssetsNet	41,432us-gaap_DeferredTaxAssetsNet
Depreciation	(16,226)	(18,669)
Amortization	(7,001)	(7,371)
Capitalized interest expense	(141)	(184)
Mine development	(571)	(2,309)
Derivative instruments and hedging activities	(1,250)	(130)
Total deferred tax liabilities	(25,189)us-gaap_DeferredTaxLiabilities	(28,663)us-gaap_DeferredTaxLiabilities
Net deferred tax asset	$ 30,507us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 12,769us-gaap_DeferredTaxAssetsLiabilitiesNet

Deferred tax liabilities deferred expense mine development.

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