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Property, Plant, and Equipment, net Narrative (Details) (USD $)
3 Months Ended 6 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Mar. 31, 2015
Mar. 31, 2014
Sep. 30, 2014
Property, Plant and Equipment [Line Items]            
Gain from change in estimate on ARO obligation   $ 2,900,000us-gaap_AssetRetirementObligationRevisionOfEstimate   $ 2,879,000us-gaap_AssetRetirementObligationRevisionOfEstimate    
Asset retirement obligation, revision in estimated cash flows 0emkr_AssetRetirementObligationGainLossfromRevisionofEstimates 800,000emkr_AssetRetirementObligationGainLossfromRevisionofEstimates 0emkr_AssetRetirementObligationGainLossfromRevisionofEstimates 845,000emkr_AssetRetirementObligationGainLossfromRevisionofEstimates 0emkr_AssetRetirementObligationGainLossfromRevisionofEstimates  
Accumulated depreciation 22,600,000us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment     22,600,000us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment   21,500,000us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment
Leasehold Improvements            
Property, Plant and Equipment [Line Items]            
Decrease in property, plant, and equipment   $ (2,100,000)us-gaap_PropertyPlantAndEquipmentTransfersAndChanges
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LeaseholdImprovementsMember