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Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Significant Accounting Policies [Line Items]      
Checks outstanding $ 7,039,000us-gaap_AccountsPayableOtherCurrent $ 3,389,000us-gaap_AccountsPayableOtherCurrent  
Transfers into Level 2 0us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount 0us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount  
Transfers out of Level 2 0us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount 0us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount  
Proceeds from sale and maturity of available for sale securities 22,959,000us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities 25,263,000us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities 29,767,000us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities
Net unrealized losses included OCI 17,000us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss 70,000us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss 30,000us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
Contractual maturities of marketable securities, within one year 3,523,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue    
Contractual maturities of marketable securities, beyond one year to five years 6,152,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue    
Contractual maturities of marketable securities, beyond five years to ten years 8,108,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue    
Contractual maturities of marketable securities, beyond ten years 4,621,000us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue    
Goodwill 11,485,000us-gaap_Goodwill 11,485,000us-gaap_Goodwill  
Goodwill impairment charges 0us-gaap_GoodwillImpairmentLoss 2,840,000us-gaap_GoodwillImpairmentLoss 0us-gaap_GoodwillImpairmentLoss
Change in contingent consideration liability   3,000,000us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1  
Goodwill, Cumulative Impairment Charges 2,840,000us-gaap_GoodwillImpairedAccumulatedImpairmentLoss    
Impairment of finite lived intangible assets 2,063,000us-gaap_ImpairmentOfIntangibleAssetsFinitelived    
Gross carrying amount of intangibles 21,968,000us-gaap_FiniteLivedIntangibleAssetsGross 26,142,000us-gaap_FiniteLivedIntangibleAssetsGross  
Accumulated amortization of intangibles 11,324,000us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization 1,716,000us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization  
Amortization expense 11,991,000us-gaap_AmortizationOfIntangibleAssets 667,000us-gaap_AmortizationOfIntangibleAssets 1,049,000us-gaap_AmortizationOfIntangibleAssets
Advertising expense 202,000us-gaap_AdvertisingExpense 363,000us-gaap_AdvertisingExpense 210,000us-gaap_AdvertisingExpense
Defense Products [Member]      
Significant Accounting Policies [Line Items]      
Goodwill 11,485,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= npk_DefenseProductsMember		 11,485,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= npk_DefenseProductsMember		  
Absorbent Products [Member]      
Significant Accounting Policies [Line Items]      
Construction in progress 8,100,000us-gaap_ConstructionInProgressGross
/ us-gaap_StatementBusinessSegmentsAxis
= npk_AbsorbentProductsMember		    
Housewares/ Small Appliances [Member]      
Significant Accounting Policies [Line Items]      
Standard product warranty coverage period 60 days    
Sales returns coverage period 60 days    
Minimum [Member]      
Significant Accounting Policies [Line Items]      
Accounts receivable, collection period 30 days    
Economic use period for intangibles 1 year    
Minimum [Member] | Housewares/ Small Appliances [Member]      
Significant Accounting Policies [Line Items]      
Standard product warranty coverage period 1 year    
Minimum [Member] | Buildings [Member]      
Significant Accounting Policies [Line Items]      
Useful life 15 years    
Minimum [Member] | Machinery and Equipment [Member]      
Significant Accounting Policies [Line Items]      
Useful life 3 years    
Minimum [Member] | Land Improvements [Member]      
Significant Accounting Policies [Line Items]      
Useful life 15 years    
Maximum [Member]      
Significant Accounting Policies [Line Items]      
Accounts receivable, collection period 60 days    
Economic use period for intangibles 9 years    
Maximum [Member] | Housewares/ Small Appliances [Member]      
Significant Accounting Policies [Line Items]      
Standard product warranty coverage period 12 years    
Maximum [Member] | Buildings [Member]      
Significant Accounting Policies [Line Items]      
Useful life 40 years    
Maximum [Member] | Machinery and Equipment [Member]      
Significant Accounting Policies [Line Items]      
Useful life 10 years    
Maximum [Member] | Land Improvements [Member]      
Significant Accounting Policies [Line Items]      
Useful life 20 years    
Maximum [Member] | Variable Rate Demand Notes [Member]      
Significant Accounting Policies [Line Items]      
Number of days to tender securites 7 days    
Government Sales Contract Intangible Assets [Member]      
Significant Accounting Policies [Line Items]      
Gross carrying amount of intangibles 21,690,000us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= npk_GovernmentSalesContractIntangibleAssetsMember		 21,690,000us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= npk_GovernmentSalesContractIntangibleAssetsMember		  
Other Intangible Assets [Member]      
Significant Accounting Policies [Line Items]      
Gross carrying amount of intangibles $ 278,000us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_OtherIntangibleAssetsMember		 $ 4,452,000us-gaap_FiniteLivedIntangibleAssetsGross
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_OtherIntangibleAssetsMember