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Concentrations (Narrative) (Details)
12 Months Ended
Dec. 31, 2014
customer
Dec. 31, 2013
customer
Dec. 31, 2012
customer
Concentration Risk [Line Items]      
Number of entity empolyees, union members 163npk_NumberOfEntityEmpolyeesUnionMembers    
Percentage of entity employees, union members 16.00%npk_PercentageOfEntityEmployeesUnionMembers    
Absorbent Products [Member]      
Concentration Risk [Line Items]      
Major customers contributing to net sales   0npk_MajorCustomersContributingToNetSales
/ us-gaap_StatementBusinessSegmentsAxis
= npk_AbsorbentProductsMember
 
Net sales, major customer, percentage   10.00%npk_NetSalesMajorCustomerPercentage
/ us-gaap_StatementBusinessSegmentsAxis
= npk_AbsorbentProductsMember
 
Housewares/ Small Appliances [Member]      
Concentration Risk [Line Items]      
Major customers contributing to net sales 1npk_MajorCustomersContributingToNetSales
/ us-gaap_StatementBusinessSegmentsAxis
= npk_HousewaresSmallAppliancesMember
0npk_MajorCustomersContributingToNetSales
/ us-gaap_StatementBusinessSegmentsAxis
= npk_HousewaresSmallAppliancesMember
1npk_MajorCustomersContributingToNetSales
/ us-gaap_StatementBusinessSegmentsAxis
= npk_HousewaresSmallAppliancesMember
Net sales, major customer, percentage 10.00%npk_NetSalesMajorCustomerPercentage
/ us-gaap_StatementBusinessSegmentsAxis
= npk_HousewaresSmallAppliancesMember
10.00%npk_NetSalesMajorCustomerPercentage
/ us-gaap_StatementBusinessSegmentsAxis
= npk_HousewaresSmallAppliancesMember
10.00%npk_NetSalesMajorCustomerPercentage
/ us-gaap_StatementBusinessSegmentsAxis
= npk_HousewaresSmallAppliancesMember