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

Number of vessels in which time charter revenues are derived for next twelve months.

No definition available.

Name:	nat_NumberOfVesselsInWhichTimeCharterRevenuesAreDerivedForNextTwelveMonths
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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Number of vessels in which time charter revenues are derived for subsequent years.

No definition available.

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

Refers to the term of voyage contracts, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

No definition available.

Name:	nat_TermOfVoyageContracts
Namespace Prefix:	nat_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Amount, before accumulated amortization and accumulated impairment loss, of asset recognized from cost incurred to obtain or fulfill contract with customer.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 340
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-Name Accounting Standards Codification
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-Paragraph 3
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-URI https://asc.fasb.org//1943274/2147479483/340-40-50-3

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- References

No definition available.

Name:	us-gaap_DisaggregationOfRevenueAbstract
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Data Type:	xbrli:stringItemType
Balance Type:	na
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- References

No definition available.

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Amount of lease payments to be received by lessor for operating lease.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
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Amount of lease payment to be received by lessor for operating lease in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of lease payment to be received by lessor for operating lease in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount of lease payment to be received by lessor for operating lease in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Data Type:	xbrli:monetaryItemType
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Amount of lease payment to be received by lessor for operating lease in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 924
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
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Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
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