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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of decrease in accumulated depreciation as a result of sale or disposal of vessel and equipment.

No definition available.

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Data Type:	xbrli:monetaryItemType
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Amount before accumulated depreciation of vessels used in the normal conduct of business and not intended for resale.

No definition available.

Name:	nat_VesselsGross
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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Carrying amount at the balance sheet date of vessels used in the normal conduct of business and not intended for resale, net of accumulated depreciation on vessels.

No definition available.

Name:	nat_VesselsNet
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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

The number of vessels under the operational management at of the balance sheet date.

No definition available.

Name:	nat_VesselsUnderOperations
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Data Type:	xbrli:integerItemType
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Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
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-Section 50
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-Publisher FASB
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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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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The aggregate amount of write-downs for impairments recognized during the period for long-lived assets held for abandonment, exchange or sale.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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- References

No definition available.

Name:	us-gaap_PropertyPlantAndEquipmentAbstract
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Amount of acquisition of long-lived, physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

No definition available.

Name:	us-gaap_PropertyPlantAndEquipmentAdditions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of divestiture of long-lived, physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

No definition available.

Name:	us-gaap_PropertyPlantAndEquipmentDisposals
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

Name:	us-gaap_PropertyPlantAndEquipmentNetAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The estimated or actual value of the asset at the end of its useful life or when it is no longer serviceable (cannot be used for its original purpose).

No definition available.

Name:	us-gaap_PropertyPlantAndEquipmentSalvageValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

No definition available.