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Note 4 - Discontinued Operations (Details Textual) - USD ($) $ in Thousands		12 Months Ended
	Dec. 26, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Escrow Deposits Related to Property Sales				$ 4,465
Atchison Facility [Member]
Disposal Group, Including Discontinued Operation, Consideration	$ 37,200
Atchison Facility [Member] | Placed in Escrow Unitil February 2018 [Member]
Escrow Deposits Related to Property Sales	800
Atchison Facility [Member] | Placed in Escrow Until December 2018 [Member]
Escrow Deposits Related to Property Sales	$ 3,700

X

- Definition

Amount of consideration received or receivable for the disposal of assets and liabilities, including discontinued operation.

+ References

No definition available.

+ Details

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

X

- Definition

Escrow deposits related to property sales in noncash investing and financing activities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586

+ Details

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

X

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=nwpx_AtchisonFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DisposalGroupClassificationAxis=nwpx_PlacedInEscrowUnitilFebruary2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DisposalGroupClassificationAxis=nwpx_PlacedInEscrowUntilDecember2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: