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DEFERRED COMPENSATION LIABILITY (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Post Retirement Benefits With Individual Employees [Line Items]      
Total expense recognized $ 782us-gaap_DeferredCompensationArrangementWithIndividualCompensationExpense $ 832us-gaap_DeferredCompensationArrangementWithIndividualCompensationExpense $ 1,391us-gaap_DeferredCompensationArrangementWithIndividualCompensationExpense
Executive Vice President and Chief Financial Officer [Member]      
Post Retirement Benefits With Individual Employees [Line Items]      
Required service period 20 years 6 months    
Maximum benefit as a percentage of salary (in hundredths) 66.00%nat_DeferredCompensationLiabilityMaximumBenefitAsPercentageOfSalary
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefFinancialOfficerMember		    
Retirement age 67 years    
Imputed interest rate (in hundredths) 2.30%nat_DeferredCompensationLiabilityImputedInterest
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefFinancialOfficerMember		 4.00%nat_DeferredCompensationLiabilityImputedInterest
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefFinancialOfficerMember		  
Chairman, President and CEO [Member]      
Post Retirement Benefits With Individual Employees [Line Items]      
Required service period 14 years    
Maximum benefit as a percentage of salary (in hundredths) 66.00%nat_DeferredCompensationLiabilityMaximumBenefitAsPercentageOfSalary
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefExecutiveOfficerMember		    
Retirement age 70 years    
Imputed interest rate (in hundredths) 2.30%nat_DeferredCompensationLiabilityImputedInterest
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefExecutiveOfficerMember		 4.00%nat_DeferredCompensationLiabilityImputedInterest
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefExecutiveOfficerMember