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Business Acquisition (Schedule of Pro Forma Results of Operations) (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
3 Months Ended
Apr. 05, 2015
Mar. 30, 2014
Business Acquisition [Line Items]    
Basic and diluted 6,943us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted 6,921us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted
Tech Ord [Member]    
Business Acquisition [Line Items]    
Net Sales   87,189us-gaap_BusinessAcquisitionsProFormaRevenue
/ us-gaap_BusinessAcquisitionAxis
= npk_TechOrdMember
Net Earnings   4,410us-gaap_BusinessAcquisitionsProFormaIncomeLossFromContinuingOperationsBeforeChangesInAccountingAndExtraordinaryItemsNetOfTax
/ us-gaap_BusinessAcquisitionAxis
= npk_TechOrdMember
Net earnings per share (basic and diluted)   0.64npk_BusinessAcquisitionProFormaEarningsPerShareBasicAndDiluted
/ us-gaap_BusinessAcquisitionAxis
= npk_TechOrdMember
Basic and diluted   6,921us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted
/ us-gaap_BusinessAcquisitionAxis
= npk_TechOrdMember