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Property, Plant, and Equipment - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Jun. 30, 2014
Property, Plant and Equipment [Line Items]      
Depreciation expense $ 144us-gaap_Depreciation $ 132us-gaap_Depreciation  
Lease back transaction lease term Five year term    
Blade Mold [Member]      
Property, Plant and Equipment [Line Items]      
Cost of machinery disposed 61us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= nrthf_BladeMoldMember
   
Net book value of machinery disposed 50nrthf_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipmentNetBookValue
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= nrthf_BladeMoldMember
   
Significant Unobservable Inputs (Level 3) [Member]      
Property, Plant and Equipment [Line Items]      
Current fair value of the facility held for sale     $ 1,300us-gaap_AssetsHeldForSaleLongLivedFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member