The percentage of specified real property securing the loan that the debt must not exceed according to covenant.

No definition available.

Name:	nwpx_DebtInstrumentCovenantMaximumDebtPercentageOfSecuringProperty
Namespace Prefix:	nwpx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of excess cash flow that repayment may be required under debt instrument covenant

+ References

No definition available.

+ Details

Name:	nwpx_DebtInstrumentCovenantRepaymentPercentageOfExcessCashFlow
Namespace Prefix:	nwpx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The fraction of original principal amount that is required periodic payments.

+ References

No definition available.

+ Details

Name:	nwpx_DebtInstrumentPeriodicPaymentFractionOfOriginalPrincipalAmount
Namespace Prefix:	nwpx_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

The maximum senior leverage ratio.

+ References

No definition available.

+ Details

Name:	nwpx_MaximumSeniorLeverageRatio
Namespace Prefix:	nwpx_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

The minimum fixed charge coverage ratio.

+ References

No definition available.

+ Details

Name:	nwpx_MinimumFixedChargeCoverageRatio
Namespace Prefix:	nwpx_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of voting equity interests acquired at the acquisition date in the business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

+ Details

Name:	us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=116868678&loc=d3e1043-128460

+ Details

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

- Definition

Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6613-128477

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6578-128477

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
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-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LineOfCreditFacilityAxis=nwpx_WellsFargoBankNaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
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- Details

Name:	srt_RangeAxis=srt_MaximumMember
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- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
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- Details

Name:	us-gaap_DebtInstrumentAxis=nwpx_DelayedDrawTermLoanTermLoanMember
Namespace Prefix:
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- Details

Name:	us-gaap_BusinessAcquisitionAxis=nwpx_GenevaPipeCompanyIncGenevaMember
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Note 20 - Subsequent Events (Details Textual) - USD ($) $ in Thousands			12 Months Ended
	Jan. 31, 2020	Oct. 25, 2018	Dec. 31, 2019	Dec. 31, 2018
Revolving Credit Facility [Member] \| Wells Fargo Bank, N.A. [Member]
Line of Credit Facility, Maximum Borrowing Capacity		$ 60,000
Long-term Line of Credit, Total			$ 0	$ 11,500
Line of Credit Facility, Remaining Borrowing Capacity			40,700
Revolving Credit Facility [Member] \| Wells Fargo Bank, N.A. [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Minimum [Member]
Debt Instrument, Basis Spread on Variable Rate		1.50%
Revolving Credit Facility [Member] \| Wells Fargo Bank, N.A. [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Maximum [Member]
Debt Instrument, Basis Spread on Variable Rate		2.00%
Subsequent Event [Member] \| Wells Fargo Bank, N.A. [Member] \| Delayed Draw Term Loan ("Term Loan") [Member]
Debt Instrument, Face Amount	$ 16,000
Debt Instrument, Periodic Payment, Fraction of Original Principal Amount	0.0167
Debt Instrument, Covenant, Maximum Debt Percentage of Securing Property	60.00%
Subsequent Event [Member] \| Wells Fargo Bank, N.A. [Member] \| Delayed Draw Term Loan ("Term Loan") [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Minimum [Member]
Debt Instrument, Basis Spread on Variable Rate	2.00%
Subsequent Event [Member] \| Revolving Credit Facility [Member] \| Wells Fargo Bank, N.A. [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 74,000
Long-term Line of Credit, Total	19,000
Line of Credit Facility, Remaining Borrowing Capacity	$ 39,000
Debt Instrument, Covenant, Repayment Percentage of Excess Cash Flow	20.00%
Maximum Senior Leverage Ratio	3
Minimum Fixed Charge Coverage Ratio	1.1
Subsequent Event [Member] \| Revolving Credit Facility [Member] \| Wells Fargo Bank, N.A. [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Maximum [Member]
Debt Instrument, Basis Spread on Variable Rate	2.50%
Geneva Pipe Company, Inc. ("Geneva") [Member]
Business Combination, Acquisition Related Costs			$ 600
Geneva Pipe Company, Inc. ("Geneva") [Member] \| Subsequent Event [Member]
Business Acquisition, Percentage of Voting Interests Acquired	100.00%
Business Combination, Consideration Transferred, Total	$ 49,400