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Summary of Significant Accounting Policies and Practices - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Aug. 29, 2014
Significant Accounting Policies [Line Items]        
Maturity period of highly liquid investments Three months or less      
Percentage of subsidiaries owned 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent  
Joint Venture [Member]        
Significant Accounting Policies [Line Items]        
Percentage of subsidiaries owned 49.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ dei_LegalEntityAxis
= us-gaap_CorporateJointVentureMember		     49.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ dei_LegalEntityAxis
= us-gaap_CorporateJointVentureMember
RFK [Member]        
Significant Accounting Policies [Line Items]        
Percentage of subsidiaries owned 99.70%us-gaap_MinorityInterestOwnershipPercentageByParent
/ us-gaap_BusinessAcquisitionAxis
= nnbr_RfkValjciciKonjicMember