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Note 5 - Leases (Details Textual)	3 Months Ended
	Mar. 31, 2019
Maximum [Member]
Lessee, Term of Contract	10 years

X

- Definition

Term of lessee's finance and operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	nwpx_LesseeTermOfContract
Namespace Prefix:	nwpx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: