Name:	nat_AccumulatedImpairmentLossOnVessels
Namespace Prefix:	nat_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Refers to be number of vessels built during the period.

No definition available.

Name:	nat_NumberOfVesselsBuilt
Namespace Prefix:	nat_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

The number of vessels during the period, which are disposed to be used in the normal conduct of business and not intended for resale.

No definition available.

Name:	nat_NumberOfVesselsDisposed
Namespace Prefix:	nat_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Number of vessels sold during the period.

No definition available.

Name:	nat_NumberOfVesselsSold
Namespace Prefix:	nat_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Amount before accumulated depreciation of vessels used in the normal conduct of business and not intended for resale.

No definition available.

Name:	nat_VesselsGross
Namespace Prefix:	nat_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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
Namespace Prefix:	nat_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

No definition available.

Name:	nat_VesselsUnderOperations
Namespace Prefix:	nat_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

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://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(8)(b))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(14))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482099/360-10-50-1

Name:	us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of gain (loss) on sale or disposal of property, plant and equipment assets, including oil and gas property and timber property.

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

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
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-28

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 360
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 15
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482130/360-10-45-15

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

- References

No definition available.

Name:	us-gaap_PropertyPlantAndEquipmentAbstract
Namespace Prefix:	us-gaap_
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.