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SHARE-BASED COMPENSATION PLAN (Details) (USD $)
In Millions, except Share data, unless otherwise specified
2 Months Ended 12 Months Ended 3 Months Ended 0 Months Ended
Feb. 28, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Person
Mar. 31, 2013
Aug. 05, 2011
Feb. 23, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Common Stock, par value (in dollars per share)   $ 0.01us-gaap_CommonStockParOrStatedValuePerShare $ 0.01us-gaap_CommonStockParOrStatedValuePerShare          
Restricted shares -Employees and Non-Employees [Roll Forward]                
Non-vested at beginning of period (in shares) 226,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber              
Fully vested shares (in shares) 193,000us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares              
Equity Incentive Plan 2011 [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Restricted shares authorized (in shares)         400,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
     
Individuals in management and Board members participating in incentive plan         23nat_IndividualsInManagementandBoardParticipatingInIncentivePlan
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
     
Percentage of allocated shares to outstanding shares (in hundredths)         0.80%nat_PercentageOfAllocatedSharesToOutstandingShares
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
     
Share-based compensation arrangement shares authorized and issued, first range (in shares)         326,000nat_ShareBasedCompensationArrangementSharesAuthorizedAndIssuedFirstRange
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
     
Share-based compensation arrangement shares authorized and issued, second range (in shares)         74,000nat_ShareBasedCompensationArrangementSharesAuthorizedAndIssuedSecondRange
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
     
Total compensation cost for the period   $ 1.1us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
$ 2.1us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
$ 1.3us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
       
Vesting requirement lifted for restricted shares     174,000nat_VestingRequirementLiftedForRestrictedShares
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
         
Restricted shares -Employees and Non-Employees [Roll Forward]                
Granted during the year (in shares)       400,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
       
Equity Incentive Plan 2011 [Member] | Minimum [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based compensation arrangement shares cliff vesting period         4 years      
Equity Incentive Plan 2011 [Member] | Maximum [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based compensation arrangement shares cliff vesting period         5 years      
Scandic American Shipping Ltd. [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Number of shares transferred in acquisition (in shares)       1,910,112us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued
/ dei_LegalEntityAxis
= nat_ScandicAmericanShippingLtdMember
       
Acquisition of shares in percentage (in hundredths)     100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
/ dei_LegalEntityAxis
= nat_ScandicAmericanShippingLtdMember
         
Employees [Member]                
Restricted shares -Employees and Non-Employees [Roll Forward]                
Non-vested at beginning of period (in shares)   163,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
163,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
    163,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
   
Granted during the year (in shares)   0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
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= nat_EmployeesMember
0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
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= nat_EmployeesMember
         
Vested during the year (in shares)   0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
         
Forfeited during the year (in shares)   (10,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
         
Non-vested at end of period (in shares)   153,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
163,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
         
Weighted-average grant-date fair value [Abstract]                
Non-vested at beginning of period (in dollars per share)   $ 23.88us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
$ 23.88us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
    $ 23.88us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
   
Granted during the year (in dollars per share)   $ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
$ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
         
Vested during the year (in dollars per share)   $ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
$ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
         
Forfeited during the year (in dollars per share)   $ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
$ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
         
Non-vested at end of period (in dollars per share)   $ 23.88us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
$ 23.88us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
         
Non-Employees [Member]                
Restricted shares -Employees and Non-Employees [Roll Forward]                
Non-vested at beginning of period (in shares)   40,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
228,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
    228,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
   
Granted during the year (in shares)   33,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
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= nat_NonEmployeesMember
0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
         
Vested during the year (in shares)   0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
(174,000)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
         
Forfeited during the year (in shares)   0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
(14,500)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
         
Non-vested at end of period (in shares)   73,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
40,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
         
Weighted-average grant-date fair value [Abstract]                
Non-vested at beginning of period (in dollars per share)   $ 24.84us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
$ 22.06us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
    $ 22.06us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
   
Granted during the year (in dollars per share)   $ 9.84us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
$ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
         
Vested during the year (in dollars per share)   $ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
$ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
         
Forfeited during the year (in dollars per share)   $ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
$ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
         
Non-vested at end of period (in dollars per share)   $ 18.06us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
$ 24.84us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_NonEmployeesMember
         
Employees and Non Employees [Member] | Equity Incentive Plan 2011 [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Shares held in treasury or repurchase of restricted common shares added to treasury (in shares)   10,000us-gaap_TreasuryStockSharesAcquired
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesAndNonEmployeesMember
14,500us-gaap_TreasuryStockSharesAcquired
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesAndNonEmployeesMember
8,500us-gaap_TreasuryStockSharesAcquired
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesAndNonEmployeesMember
       
Restricted shares remaining in plan     203,000nat_ShareBasedCompensationArrangementsByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingNumber
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesAndNonEmployeesMember
         
Restricted Stock Awards [Member] | Scandic American Shipping Ltd. [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Manager's right to ownership of company's total outstanding shares (in hundredths)       2.00%nat_DueToRelatedPartiesManagersRightToOwnershipOfCompanysTotalOutstandingShares
/ dei_LegalEntityAxis
= nat_ScandicAmericanShippingLtdMember
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
       
Common Stock, par value (in dollars per share)       $ 0.01us-gaap_CommonStockParOrStatedValuePerShare
/ dei_LegalEntityAxis
= nat_ScandicAmericanShippingLtdMember
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
       
Lock up period of new shares issued to the Manager       1 year        
Restricted shares -Employees and Non-Employees [Roll Forward]                
Granted during the year (in shares)       112,245us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ dei_LegalEntityAxis
= nat_ScandicAmericanShippingLtdMember
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
       
Restricted Stock Awards [Member] | Scandic American Shipping Ltd. [Member] | Equity Incentive Plan 2011 [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Fair value of shares issued to manager (in dollars per share)       $ 13.73nat_ShareBasedCompensationFairValueOfSharesIssued
/ dei_LegalEntityAxis
= nat_ScandicAmericanShippingLtdMember
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
       
Total compensation cost for the period           1.1us-gaap_AllocatedShareBasedCompensationExpense
/ dei_LegalEntityAxis
= nat_ScandicAmericanShippingLtdMember
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
   
Restricted Stock Awards [Member] | Employees [Member] | Equity Incentive Plan 2011 [Member]                
Restricted shares -Employees and Non-Employees [Roll Forward]                
Granted during the year (in shares)   33,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesMember
           
Restricted Stock Awards [Member] | Employees and Non Employees [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Shares held in treasury or repurchase of restricted common shares added to treasury (in shares)     33,000us-gaap_TreasuryStockSharesAcquired
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesAndNonEmployeesMember
         
Restricted Stock Awards [Member] | Employees and Non Employees [Member] | Equity Incentive Plan 2011 [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Fair value of shares issued to manager (in dollars per share)             $ 18.05nat_ShareBasedCompensationFairValueOfSharesIssued
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesAndNonEmployeesMember
$ 23.88nat_ShareBasedCompensationFairValueOfSharesIssued
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesAndNonEmployeesMember
Common shares repurchased at fair value (in shares)   10,000nat_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvested
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesAndNonEmployeesMember
           
Unrecognized compensation cost related to unvested restricted stock   0.4us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesAndNonEmployeesMember
           
Weighted average period to recognize the unrecognized compensation cost   0 years 7 months 6 days            
Fair value of shares vested   $ 1.7us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesAndNonEmployeesMember
           
Restricted shares -Employees and Non-Employees [Roll Forward]                
Granted during the year (in shares)             74,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesAndNonEmployeesMember
326,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_PlanNameAxis
= nat_EquityIncentivePlan2011Member
/ nat_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAwardTypeAxis
= us-gaap_RestrictedStockMember
/ nat_ShareBasedCompensationArrangementWithIndividualShareBasedPaymentsByTitleOfIndividualAxis
= nat_EmployeesAndNonEmployeesMember