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Related Party transactions - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014
Eagle Asset Management, Inc. [Member] | Minimum [Member] |
Related Party Transaction [Line Items] |
Company's shares owned by related party | 10.00%nwpx_RelatedPartyTransactionPercentageOwnedByRelatedParty / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = nwpx_EagleAssetManagementIncMember
Raymond James & Associates [Member] |
Related Party Transaction [Line Items] |
Related party transaction amount | 1.2 us-gaap_RelatedPartyTransactionAmountsOfTransaction / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = nwpx_RaymondJamesAndAssociatesMember

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- Definition Related party transaction percentage owned by related party. + References No definition available. + Details
Name: | nwpx_RelatedPartyTransactionPercentageOwnedByRelatedParty
Namespace Prefix: | nwpx_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of transactions with related party during the financial reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39622-107864 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39603-107864 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864 + Details
Name: | us-gaap_RelatedPartyTransactionAmountsOfTransaction
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_RelatedPartyTransactionLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=nwpx_EagleAssetManagementIncMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=nwpx_RaymondJamesAndAssociatesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |