CONSOLIDATED STATEMENTS OF OPERATIONS (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
CONSOLIDATED STATEMENTS OF OPERATIONS [Abstract]
Voyage Revenues	$ 351,049us-gaap_CargoAndFreightRevenue	$ 243,657us-gaap_CargoAndFreightRevenue	$ 130,682us-gaap_CargoAndFreightRevenue
Voyage Expenses	(199,430)us-gaap_CostOfServices	(173,410)us-gaap_CostOfServices	(38,670)us-gaap_CostOfServices
Voyage Operating Expenses - Excluding Depreciation Expense Presented Below	(62,500)us-gaap_OperatingCostsAndExpenses	(64,924)us-gaap_OperatingCostsAndExpenses	(63,965)us-gaap_OperatingCostsAndExpenses
General and Administrative Expenses	(14,863)us-gaap_GeneralAndAdministrativeExpense	(19,555)us-gaap_GeneralAndAdministrativeExpense	(14,700)us-gaap_GeneralAndAdministrativeExpense
Depreciation Expenses	(80,531)us-gaap_Depreciation	(74,375)us-gaap_Depreciation	(69,219)us-gaap_Depreciation
Impairment Loss on Vessel	0us-gaap_AssetImpairmentCharges	0us-gaap_AssetImpairmentCharges	(12,030)us-gaap_AssetImpairmentCharges
Loss on Contract	0us-gaap_GainLossOnContractTermination	(5,000)us-gaap_GainLossOnContractTermination	0us-gaap_GainLossOnContractTermination
Fees for provided services	1,500us-gaap_SalesRevenueServicesNet	0us-gaap_SalesRevenueServicesNet	0us-gaap_SalesRevenueServicesNet
Net Operating Loss	(4,775)us-gaap_OperatingIncomeLoss	(93,608)us-gaap_OperatingIncomeLoss	(67,902)us-gaap_OperatingIncomeLoss
Interest Income	181nat_InterestIncome	146nat_InterestIncome	357nat_InterestIncome
Interest Expenses	(12,244)us-gaap_InterestExpense	(11,518)us-gaap_InterestExpense	(5,854)us-gaap_InterestExpense
Dividends Received	252us-gaap_InvestmentIncomeDividend	0us-gaap_InvestmentIncomeDividend	0us-gaap_InvestmentIncomeDividend
Gain on shares	3,286nat_GainLossOnSaleOfInvestmentShares	0nat_GainLossOnSaleOfInvestmentShares	0nat_GainLossOnSaleOfInvestmentShares
Other Financial Income (Expenses)	(1,126)us-gaap_OtherNonoperatingIncomeExpense	(391)us-gaap_OtherNonoperatingIncomeExpense	207us-gaap_OtherNonoperatingIncomeExpense
Total Other Expenses	(9,651)us-gaap_NonoperatingIncomeExpense	(11,763)us-gaap_NonoperatingIncomeExpense	(5,290)us-gaap_NonoperatingIncomeExpense
Net Loss Before Income Taxes and Equity Income	(14,426)	(105,371)	(73,192)
Income Tax Expense	(47)us-gaap_IncomeTaxExpenseBenefit	(86)us-gaap_IncomeTaxExpenseBenefit	0us-gaap_IncomeTaxExpenseBenefit
Equity Income	1,665us-gaap_GainLossOnInvestments	40us-gaap_GainLossOnInvestments	0us-gaap_GainLossOnInvestments
Net Loss	$ (12,808)us-gaap_NetIncomeLoss	$ (105,417)us-gaap_NetIncomeLoss	$ (73,192)us-gaap_NetIncomeLoss
Basic and Diluted Loss per Share (in dollars per share)	$ (0.15)us-gaap_EarningsPerShareBasicAndDiluted	$ (1.64)us-gaap_EarningsPerShareBasicAndDiluted	$ (1.39)us-gaap_EarningsPerShareBasicAndDiluted
Basic and Diluted Average Number of Common Shares Outstanding (in shares)	85,401,179	64,101,923	52,547,623
Cash Dividends per share (in dollars per share)	$ 0.63	$ 0.64	$ 1.20

This item represents the net total realized gain (loss) of sale of investment shares during the period.

No definition available.

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Interest income derived from the cash amount on the bank accounts.

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- Definition

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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- Definition

Revenue from transporting cargo and freight between locations.

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- Definition

Aggregate dividends paid during the period for each share of common stock outstanding.

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- Definition

Total costs related to services rendered by an entity during the reporting period.

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- Definition

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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- Definition

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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No definition available.

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- Definition

Gain (loss) related to the termination of a contract between the parties. The termination may be due to many causes including early termination of a lease by a lessee, a breach of contract by one party, or a failure to perform.

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This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

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- Definition

The aggregate total of expenses of managing and administering the affairs of an entity, including affiliates of the reporting entity, which are not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

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This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses from ongoing operations, after income or loss from equity method investments, but before income taxes, extraordinary items, and noncontrolling interest.

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- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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- Definition

Amount of the cost of borrowed funds accounted for as interest expense.

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- Definition

Amount of dividend income on nonoperating securities.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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The aggregate amount of income or expense from ancillary business-related activities (that is to say, excluding major activities considered part of the normal operations of the business).

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- Definition

Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Excludes Selling, General and Administrative Expense.

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No definition available.

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The net result for the period of deducting operating expenses from operating revenues.

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No definition available.

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The net amount of other income and expense amounts, the components of which are not separately disclosed on the income statement, resulting from ancillary business-related activities (that is, excluding major activities considered part of the normal operations of the business) also known as other nonoperating income (expense) recognized for the period. Such amounts may include: (a) dividends, (b) interest on securities, (c) net gains or losses on securities, (d) unusual costs, (e) gains or losses on foreign exchange transactions, and (f) miscellaneous other income and expense items.

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Aggregate revenue during the period from services rendered in the normal course of business, after deducting allowances and discounts.

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Average number of shares or units issued and outstanding that are used in calculating basic and diluted earnings per share (EPS).

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No definition available.

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