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CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (USD $)
In Thousands, except Share data, unless otherwise specified
Common Stock [Member]
USD ($)
Treasury Shares [Member]
Additional Paid-in Capital [Member]
USD ($)
Contributed Surplus [Member]
USD ($)
Accumulated Other Comprehensive Deficit [Member]
USD ($)
Accumulated Deficit [Member]
USD ($)
Total
USD ($)
Balance at Dec. 31, 2011 $ 473us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  $ 12,867us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
$ 926,733us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
$ (212)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
$ (72,298)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
$ 867,563us-gaap_StockholdersEquity
Balance (in shares) at Dec. 31, 2011 47,303,394us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
           
Increase (decrease) in Shareholders' Equity [Roll Forward]              
Accumulated coverage of loss 0us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
(72,298)us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
72,298us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
0us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
Net Loss 0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
(73,192)us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
(73,192)us-gaap_NetIncomeLoss
Common Shares Issued, net of issuance costs (in shares) 5,500,000us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
          5,500,000us-gaap_StockIssuedDuringPeriodSharesNewIssues
Common Shares Issued, net of issuance costs 55us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  75,527us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
75,582us-gaap_StockIssuedDuringPeriodValueNewIssues
Reduction of share premium 0nat_ReductionOfSharePremium
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  (75,577)nat_ReductionOfSharePremium
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
75,577nat_ReductionOfSharePremium
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0nat_ReductionOfSharePremium
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0nat_ReductionOfSharePremium
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
0nat_ReductionOfSharePremium
Other Comprehensive (Loss) Income 0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
128us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
128us-gaap_OtherComprehensiveIncomeLossNetOfTax
Compensation - Restricted Shares (in shares) 112,245us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
           
Compensation - Restricted Shares 1us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  1,540us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
1,541us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross
Common Shares Repurchased, Equity Incentive Plan (in shares) (8,500)us-gaap_TreasuryStockSharesAcquired
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
8,500us-gaap_TreasuryStockSharesAcquired
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
         
Common Shares Repurchased, Equity Incentive Plan 0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
0nat_StockRepurchasedHeldInTreasuryValue
Share-Based Compensation (in shares) 0us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationGross
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
           
Share-Based Compensation 0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  1,258us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
1,258us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
Return of Capital 0us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
(63,497)us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
(63,497)us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
Balance at Dec. 31, 2012 529us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  15,615us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
866,515us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
(84)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
(73,192)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
809,383us-gaap_StockholdersEquity
Balance (in shares) at Dec. 31, 2012 52,907,139us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
8,500us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
         
Increase (decrease) in Shareholders' Equity [Roll Forward]              
Accumulated coverage of loss 0us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
(73,192)us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
73,192us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
0us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
Net Loss 0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
(105,417)us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
(105,417)us-gaap_NetIncomeLoss
Common Shares Issued, net of issuance costs (in shares) 20,556,250us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
          20,556,250us-gaap_StockIssuedDuringPeriodSharesNewIssues
Common Shares Issued, net of issuance costs 206us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  172,405us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
172,611us-gaap_StockIssuedDuringPeriodValueNewIssues
Other Comprehensive (Loss) Income 0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
(76)us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
(76)us-gaap_OtherComprehensiveIncomeLossNetOfTax
Common Shares Issued in connection with the Scandic acquisition (in shares) 1,910,112us-gaap_StockIssuedDuringPeriodSharesAcquisitions
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
           
Common Shares Issued in connection with the Scandic acquisition 19us-gaap_StockIssuedDuringPeriodValueAcquisitions
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  18,127us-gaap_StockIssuedDuringPeriodValueAcquisitions
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_StockIssuedDuringPeriodValueAcquisitions
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_StockIssuedDuringPeriodValueAcquisitions
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_StockIssuedDuringPeriodValueAcquisitions
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
18,146us-gaap_StockIssuedDuringPeriodValueAcquisitions
Compensation - Restricted Shares (in shares) 1,910,112us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
           
Common Shares Repurchased, Equity Incentive Plan (in shares) (14,500)us-gaap_TreasuryStockSharesAcquired
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
14,500us-gaap_TreasuryStockSharesAcquired
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
         
Common Shares Repurchased, Equity Incentive Plan 0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
0nat_StockRepurchasedHeldInTreasuryValue
Share-Based Compensation (in shares) 0us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationGross
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
           
Share-Based Compensation 0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  2,093us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
2,093us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
Return of Capital 0us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
(41,756)us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
(41,756)us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
Balance at Dec. 31, 2013 754us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  208,240us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
751,567us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
(160)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
(105,417)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
854,984us-gaap_StockholdersEquity
Balance (in shares) at Dec. 31, 2013 75,359,001us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
23,000us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
         
Increase (decrease) in Shareholders' Equity [Roll Forward]              
Accumulated coverage of loss 0us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
(105,417)us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
105,417us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
0us-gaap_AdjustmentsToAdditionalPaidInCapitalOther
Net Loss 0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
(12,808)us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
(12,808)us-gaap_NetIncomeLoss
Common Shares Issued, net of issuance costs (in shares) 13,800,000us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
          13,800,000us-gaap_StockIssuedDuringPeriodSharesNewIssues
Common Shares Issued, net of issuance costs 138us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  113,295us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
113,433us-gaap_StockIssuedDuringPeriodValueNewIssues
Reduction of share premium 0nat_ReductionOfSharePremium
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  (208,240)nat_ReductionOfSharePremium
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
208,240nat_ReductionOfSharePremium
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0nat_ReductionOfSharePremium
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0nat_ReductionOfSharePremium
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
0nat_ReductionOfSharePremium
Other Comprehensive (Loss) Income 0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
(7,872)us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
(7,872)us-gaap_OtherComprehensiveIncomeLossNetOfTax
Common Shares Repurchased, Equity Incentive Plan (in shares) (10,000)us-gaap_TreasuryStockSharesAcquired
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
10,000us-gaap_TreasuryStockSharesAcquired
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
         
Common Shares Repurchased, Equity Incentive Plan 0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  (99)nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0nat_StockRepurchasedHeldInTreasuryValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
(99)nat_StockRepurchasedHeldInTreasuryValue
Common Shares Distributed - Equity Incentive Plan (in shares) 33,000us-gaap_StockIssuedDuringPeriodSharesEmployeeStockOwnershipPlan
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
(33,000)us-gaap_StockIssuedDuringPeriodSharesEmployeeStockOwnershipPlan
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
         
Common Shares Distributed - Equity Incentive Plan 0us-gaap_StockIssuedDuringPeriodValueEmployeeStockOwnershipPlan
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0us-gaap_StockIssuedDuringPeriodValueEmployeeStockOwnershipPlan
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_StockIssuedDuringPeriodValueEmployeeStockOwnershipPlan
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_StockIssuedDuringPeriodValueEmployeeStockOwnershipPlan
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_StockIssuedDuringPeriodValueEmployeeStockOwnershipPlan
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
0us-gaap_StockIssuedDuringPeriodValueEmployeeStockOwnershipPlan
Share-Based Compensation (in shares) 0us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationGross
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
           
Share-Based Compensation 0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  1,096us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
1,096us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation
Return of Capital 0us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  0us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
(66,658)us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
0us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
0us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
(66,658)us-gaap_AdjustmentsToAdditionalPaidInCapitalDividendsInExcessOfRetainedEarnings
Balance at Dec. 31, 2014 $ 892us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
  $ 114,291us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
$ 787,732us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_OtherAdditionalCapitalMember
$ (8,032)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
$ (12,808)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
$ 882,075us-gaap_StockholdersEquity
Balance (in shares) at Dec. 31, 2014 89,182,001us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
0us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember