XML 82 R84.htm IDEA: XBRL DOCUMENT v2.4.1.9

Related Party transactions - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	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	1.2us-gaap_RelatedPartyTransactionAmountsOfTransaction / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = nwpx_RaymondJamesAndAssociatesMember

X

- 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

X

- 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

X

- Details

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

X

- Details

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

X

- Details

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