XML 66 R48.htm IDEA: XBRL DOCUMENT
v3.19.1

LONG-TERM DEBT AND CURRENT PORTION OF LONG TERM DEBT (Details) $ / shares in Units, $ in Thousands |  |  |  |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 4 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 29, 2019 USD ($) | Feb. 12, 2019 USD ($) | Dec. 07, 2018 USD ($) | Sep. 07, 2018 USD ($) | Jun. 12, 2018 USD ($) | May 04, 2018 USD ($) $ / shares | Mar. 10, 2018 USD ($) | Dec. 01, 2017 USD ($) | Dec. 31, 2018 USD ($) Vessel | Oct. 31, 2018 Building | Aug. 31, 2018 Building | Jul. 31, 2018 USD ($) Vessel Building | Jun. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) Building | Jun. 30, 2018 Vessel | Apr. 16, 2019 USD ($) | Dec. 31, 2018 USD ($) Vessel Building Shipbroker | Dec. 31, 2017 USD ($) Building | Dec. 20, 2017 USD ($) | Dec. 31, 2016 | Dec. 31, 2015 USD ($) | Dec. 31, 2012 USD ($)
Line of Credit Facility [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current portion of long-term debt |  |  |  |  |  |  |  |  |  | $ 18,692 |  |  |  |  | $ 18,692 |  |  |  | $ 18,692 | $ 0 |  |  |  |
Number of new buildings | Building |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 | 3 |  |  |  |
Number of vessels delivered | Vessel |  |  |  |  |  |  |  |  |  | 2 |  |  | 8 |  |  |  |  |  |  |  |  |  |  |
Contractual Obligation [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
2019 |  |  |  |  |  |  |  |  |  | $ 320,673 |  |  |  |  | 320,673 |  |  |  | $ 320,673 |  |  |  |  |
2020 |  |  |  |  |  |  |  |  |  | 7,629 |  |  |  |  | 7,629 |  |  |  | 7,629 |  |  |  |  |
2021 |  |  |  |  |  |  |  |  |  | 7,960 |  |  |  |  | 7,960 |  |  |  | 7,960 |  |  |  |  |
2022 |  |  |  |  |  |  |  |  |  | 8,327 |  |  |  |  | 8,327 |  |  |  | 8,327 |  |  |  |  |
2023 |  |  |  |  |  |  |  |  |  | 8,711 |  |  |  |  | 8,711 |  |  |  | 8,711 |  |  |  |  |
More than 5 years |  |  |  |  |  |  |  |  |  | 87,239 |  |  |  |  | 87,239 |  |  |  | 87,239 |  |  |  |  |
Total |  |  |  |  |  |  |  |  |  | 440,540 |  |  |  |  | 440,540 |  |  |  | $ 440,540 |  |  |  |  |
Maximum [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liquidity Outlook [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
At-the-market Offering of Common Stock |  | $ 40,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revolving Credit Facility [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 375,000 |  | $ 500,000 | $ 430,000
Maturity date |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Dec. 31, 2020 |  |  |  |  |
Fair market value of vessels assessed by number of independent shipbrokers | Shipbroker |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |
Drawn amount |  |  |  |  |  |  |  |  |  | 313,400 |  |  |  |  | 313,400 |  |  |  | $ 313,400 | $ 391,600 |  |  |  |
Deferred financing costs |  |  |  |  |  |  |  |  |  | 1,700 |  |  |  |  | 1,700 |  |  |  | 1,700 |  |  |  | $ 4,600 |
Percentage of increase in margin for the period of waiver |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.00% |  |
Associated fees |  |  |  |  |  |  |  |  |  | 13,100 |  |  |  |  | 13,100 |  |  |  | 13,100 |  |  |  |  |
Security ratio after adjustment |  |  |  |  |  |  | 135.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liquidity covenant after adjustment |  |  |  |  |  |  | $ 15,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum market cap ratio after adjustment |  |  |  |  |  |  | $ 200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of increase in undrawn borrowings |  |  |  |  |  |  | 6.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of dividends that can be distributed quarterly (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 0.03 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividend distributed |  |  |  |  |  |  | $ 4,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayment of debt |  |  |  | $ 1,400 | $ 2,800 | $ 1,400 |  | $ 4,300 |  | 18,000 |  |  | $ 27,200 | $ 10,100 | 4,200 | $ 8,300 |  |  |  |  |  |  |  |
Number of vessels sold and delivered | Vessel |  |  |  |  |  |  |  |  |  |  |  |  | 6 |  |  |  | 2 |  |  |  |  |  |  |
Number of new buildings | Building |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |
Contractual Obligation [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
2019 | [1] |  |  |  |  |  |  |  |  | 313,400 |  |  |  |  | 313,400 |  |  |  | 313,400 |  |  |  |  |
2020 | [1] |  |  |  |  |  |  |  |  | 0 |  |  |  |  | 0 |  |  |  | 0 |  |  |  |  |
2021 | [1] |  |  |  |  |  |  |  |  | 0 |  |  |  |  | 0 |  |  |  | 0 |  |  |  |  |
2022 | [1] |  |  |  |  |  |  |  |  | 0 |  |  |  |  | 0 |  |  |  | 0 |  |  |  |  |
2023 | [1] |  |  |  |  |  |  |  |  | 0 |  |  |  |  | 0 |  |  |  | 0 |  |  |  |  |
More than 5 years | [1] |  |  |  |  |  |  |  |  | 0 |  |  |  |  | 0 |  |  |  | 0 |  |  |  |  |
Total | [1] |  |  |  |  |  |  |  |  | 313,400 |  |  |  |  | 313,400 |  |  |  | 313,400 |  |  |  |  |
Revolving Credit Facility [Member] | Forecast [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Drawn amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 300,000 |  |  |  |  |  |
Current portion of long-term debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 13,400 |  |  |  |  |  |
Repayment of debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 13,400 |  |  |  |  |  |
Long-term debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 300,000 |  |  |  |  |  |
Revolving Credit Facility [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum market cap to value adjusted total assets |  |  |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revolving Credit Facility [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum market cap to value adjusted total assets |  |  |  |  |  |  | 45.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of equity |  |  |  |  |  |  | $ 75,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Secured Credit Facility [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  | $ 306,100 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity date |  |  | Feb. 28, 2024 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument term |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loan amortizing, maturity period |  |  | 20 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discretionary excess cash mechanism for the lender that equals to net earnings from collateral vessels |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt covenants, minimum value in unrestricted cash |  |  | $ 30,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt covenants percentage in loan-to-vessel ratio |  |  | 70.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financing of Newbuildings [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Drawn amount |  |  |  |  |  |  |  |  |  | 127,100 |  |  |  |  | 127,100 |  |  |  | 127,100 |  |  |  |  |
Deferred financing costs |  |  |  |  |  |  |  |  |  | 2,300 |  |  |  |  | 2,300 |  |  |  | $ 2,300 |  |  |  |  |
Number of new buildings | Building |  |  |  |  |  |  |  |  |  |  | 3 | 3 | 3 |  |  |  |  |  |  |  |  |  |  |
Financing of outstanding commitments per ship |  |  |  |  |  |  |  |  | $ 39,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of purchase price expected to pay by lending provider |  |  |  |  |  |  |  |  | 77.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from vessel financing |  |  |  |  |  |  |  |  | $ 12,500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term of bareboat charter agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10 years |  |  |  |  |
Obligation to purchase the vessels |  |  |  |  |  |  |  |  | $ 13,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
First flexibility period to purchase the vessels |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 60 months |  |  |  |  |
Second flexibility period to purchase the vessels |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 84 months |  |  |  |  |
Interest rate |  |  |  |  |  |  |  |  | 7.28% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum value adjusted equity |  |  |  |  |  |  |  |  | $ 175,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum value adjusted equity ratio |  |  |  |  |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum liquidity value |  |  |  |  |  |  |  |  | $ 20,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of vessels delivered | Vessel |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 |  |  |  |  |
Long term debt current |  |  |  |  |  |  |  |  |  | 7,300 |  |  |  |  | 7,300 |  |  |  | $ 7,300 |  |  |  |  |
Contractual Obligation [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
2019 | [2] |  |  |  |  |  |  |  |  | 7,273 |  |  |  |  | 7,273 |  |  |  | 7,273 |  |  |  |  |
2020 | [2] |  |  |  |  |  |  |  |  | 7,629 |  |  |  |  | 7,629 |  |  |  | 7,629 |  |  |  |  |
2021 | [2] |  |  |  |  |  |  |  |  | 7,960 |  |  |  |  | 7,960 |  |  |  | 7,960 |  |  |  |  |
2022 | [2] |  |  |  |  |  |  |  |  | 8,327 |  |  |  |  | 8,327 |  |  |  | 8,327 |  |  |  |  |
2023 | [2] |  |  |  |  |  |  |  |  | 8,711 |  |  |  |  | 8,711 |  |  |  | 8,711 |  |  |  |  |
More than 5 years | [2] |  |  |  |  |  |  |  |  | 87,239 |  |  |  |  | 87,239 |  |  |  | 87,239 |  |  |  |  |
Total | [2] |  |  |  |  |  |  |  |  | $ 127,140 |  |  |  |  | $ 127,140 |  |  |  | $ 127,140 |  |  |  |  |
[1] | Refers to obligation to repay indebtness outstanding as of December 31, 2018 under the Credit Facility
[2] | Refers to obligation to repay indebtness outstanding as of December 31, 2018 for three newbuildings delivered in 2018

X
- Definition Refers to the amount of dividends that can be distributed quarterly under the agreement. + References No definition available. + Details
Name: | nat_AmountOfDividendsThatCanBeDistributedQuarterly
Namespace Prefix: | nat_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

X
- Definition The Company has launched an At-the-Market Offering of our common stock and believes that the current cash and cash equivalents and cash expected to be generated from operations are sufficient to meet the working capital needs and other liquidity requirements for the next 12 months. + References No definition available. + Details
Name: | nat_AtTheMarketOfferingOfCommonStock
Namespace Prefix: | nat_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The agreement contains covenants that requires a minimum value in unrestricted cash. + References No definition available. + Details
Name: | nat_DebtCovenantsMinimumValueInUnrestrictedCash
Namespace Prefix: | nat_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Percentage contains covenants that requires a maximum in loan-to-vessel value ratio. + References No definition available. + Details
Name: | nat_DebtCovenantsPercentageInLoanToVesselRatio
Namespace Prefix: | nat_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Refers to amount of minimum liquidity value of debt instrument under credit agreement. + References No definition available. + Details
Name: | nat_DebtInstrumentMinimumLiquidityValue
Namespace Prefix: | nat_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Refers to the minimum value of adjusted equity under the financing agreement. + References No definition available. + Details
Name: | nat_DebtInstrumentMinimumValueAdjustedEquity
Namespace Prefix: | nat_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The agreement contains a discretionary excess cash mechanism for the lender that equals to the net earnings from the collateral vessels, less capex provision and amortization. + References No definition available. + Details
Name: | nat_DiscretionaryExcessCashMechanismForTheLenderThatEqualsToNetEarningsFromCollateralVessels
Namespace Prefix: | nat_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of independent shipbrokers assessed fair market value of vessels. + References No definition available. + Details
Name: | nat_FairMarketValueOfVesselsAssessedByNumberOfIndependentShipbrokers
Namespace Prefix: | nat_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Refers to amount of financing of outstanding commitments of per ship for new buildings under credit agreement. + References No definition available. + Details
Name: | nat_FinancingOfOutstandingCommitmentsPerShip
Namespace Prefix: | nat_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Refers to the first flexibility period to purchase the vessels as per agreement in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | nat_FirstFlexibilityPeriodToPurchaseVessels
Namespace Prefix: | nat_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The percentage of undrawn borrowing capacity. + References No definition available. + Details
Name: | nat_LineOfCreditFacilityIncreaseInUndrawnPercentage
Namespace Prefix: | nat_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Refers to amount of liquidity covenants measure cash resources of the borrower. + References No definition available. + Details
Name: | nat_LiquidityCovenantAfterAdjustment
Namespace Prefix: | nat_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- References No definition available. + Details
Name: | nat_LiquidityOutlookAbstract
Namespace Prefix: | nat_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the loan is amortizing with a maturity profile. + References No definition available. + Details
Name: | nat_LoanAmortizingMaturityPeriod
Namespace Prefix: | nat_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Refers to amount of minimum market cap after adjustments. + References No definition available. + Details
Name: | nat_MinimumMarketCapAfterAdjustments
Namespace Prefix: | nat_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Refers to minimum market cap to value adjusted total assets for remaining duration of wavier. + References No definition available. + Details
Name: | nat_MinimumMarketCapToValueAdjustedTotalAssets
Namespace Prefix: | nat_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Refers to the minimum value of adjusted equity ratio under the financing agreement. + References No definition available. + Details
Name: | nat_MinimumValueAdjustedEquityRatio
Namespace Prefix: | nat_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Refers to be number of new buildings. + References No definition available. + Details
Name: | nat_NumberOfNewBuildings
Namespace Prefix: | nat_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Refers to number of vessels delivered to the company during the period. + References No definition available. + Details
Name: | nat_NumberOfVesselsDelivered
Namespace Prefix: | nat_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of vessels sold and delivered to new owners. + References No definition available. + Details
Name: | nat_NumberOfVesselsSoldAndDelivered
Namespace Prefix: | nat_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Obligations to purchase the vessels for a consideration value each upon the completion of the period of bareboat charter agreements. + References No definition available. + Details
Name: | nat_ObligationToPurchaseVessels
Namespace Prefix: | nat_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Refers to the percentage of increase in margin for the period of waiver. + References No definition available. + Details
Name: | nat_PercentageOfIncreaseInMarginForThePeriodOfWaiver
Namespace Prefix: | nat_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Refers to the percentage of purchase price expected to pay by lending provider under the terms of the financing agreement. + References No definition available. + Details
Name: | nat_PercentageOfPurchasePriceExpectedToPayByLendingProvider
Namespace Prefix: | nat_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of cash inflow from vessel financing, classified as financing activity. + References No definition available. + Details
Name: | nat_ProceedsFromVesselFinancing
Namespace Prefix: | nat_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Refers to the second flexibility period to purchase the vessels as per agreement in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | nat_SecondFlexibilityPeriodToPurchaseVessels
Namespace Prefix: | nat_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Refers to security ratio after adjustment coverage on financial covenants. + References No definition available. + Details
Name: | nat_SecurityRatioAfterAdjustment
Namespace Prefix: | nat_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Term of bareboat charter agreement the Company will enter into with the Lessor. + References No definition available. + Details
Name: | nat_TermOfBareboatCharterAgreement
Namespace Prefix: | nat_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments. + References No definition available. + Details
Name: | us-gaap_ContractualObligation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of contractual obligation due after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
Name: | us-gaap_ContractualObligationDueAfterFifthYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of contractual obligation due in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
Name: | us-gaap_ContractualObligationDueInFifthYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of contractual obligation due in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
Name: | us-gaap_ContractualObligationDueInFourthYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of contractual obligation due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
Name: | us-gaap_ContractualObligationDueInNextTwelveMonths
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of contractual obligation due in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
Name: | us-gaap_ContractualObligationDueInSecondYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of contractual obligation due in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
Name: | us-gaap_ContractualObligationDueInThirdYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_ContractualObligationFiscalYearMaturityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of the fee that accompanies borrowing money under the debt instrument. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentFeeAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The average effective interest rate during the reporting period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateDuringPeriod
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentTerm
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28555-108399 + Details
Name: | us-gaap_DeferredFinanceCostsNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The total amount of dividends declared in the period for each class of stock and the contractual amount of dividends (or interest on participating income bonds) that must be paid for the period (for example, unpaid cumulative dividends). Dividends declared in the current period do not include dividends declared in respect of prior-period unpaid cumulative dividends. Preferred dividends that are cumulative only if earned are deducted only to the extent that they are earned. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 66 -URI http://asc.fasb.org/extlink&oid=109260490&loc=d3e2814-109256 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60B -URI http://asc.fasb.org/extlink&oid=109260490&loc=SL5780133-109256 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60 -URI http://asc.fasb.org/extlink&oid=109260490&loc=d3e2740-109256 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60A -URI http://asc.fasb.org/extlink&oid=109260490&loc=SL5780132-109256 + Details
Name: | us-gaap_DistributedEarnings
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_LineOfCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_LineOfCreditFacilityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Date the credit facility terminates, in CCYY-MM-DD format. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityExpirationDate1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The carrying value as of the balance sheet date of the current portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(13)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(19)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LinesOfCreditCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LongTermDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19,20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_LongTermDebtCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The cash inflow from the issuance of common stock, preferred stock, treasury stock, stock options, and other types of equity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3255-108585 + Details
Name: | us-gaap_ProceedsFromIssuanceOrSaleOfEquity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585 + Details
Name: | us-gaap_RepaymentsOfLongTermDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CreditFacilityAxis=us-gaap_SecuredDebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CreditFacilityAxis=nat_NewbuildFinancingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |