Quarterly Data (Details Textual) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2012
Quarterly Data (Textual) [Abstract]
Net sales recognized under percentage of completion accounting		$ 0.8
Cost of sales reduction	$ 0.2	$ 0.1	$ 0.1	$ 0.4

Out of period adjustment to net sales using percentage of completion accounting.

No definition available.

Name:	nwpx_OutOfPeriodAdjustmentNetSales
Namespace Prefix:	nwpx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration