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Derivative Instruments (Details 3) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
MMBTU
Dec. 31, 2014
MMBTU
Natural gas contracts    
Derivative Instruments    
Outstanding derivative contracts that were entered into to hedge forecasted transactions (in mmbtu) 965,000us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_EnergyRelatedDerivativeMember		 810,000us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_EnergyRelatedDerivativeMember
Foreign exchange contracts    
Derivative Instruments    
Outstanding derivative contracts that were entered into to hedge forecasted transactions 45,869invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember		 41,237invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember