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Inventories - Components of Inventories (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Short-term inventories:
Raw materials	$ 36,715us-gaap_InventoryRawMaterialsNetOfReserves	$ 48,005us-gaap_InventoryRawMaterialsNetOfReserves
Work-in-process	1,474us-gaap_InventoryWorkInProcessNetOfReserves	1,741us-gaap_InventoryWorkInProcessNetOfReserves
Finished goods	20,746us-gaap_InventoryFinishedGoodsNetOfReserves	20,663us-gaap_InventoryFinishedGoodsNetOfReserves
Supplies	2,348us-gaap_InventorySuppliesNetOfReserves	2,370us-gaap_InventorySuppliesNetOfReserves
Total short-term inventories	61,283us-gaap_InventoryNet	72,779us-gaap_InventoryNet
Long-term inventories:
Finished goods	1,209us-gaap_InventoryNoncurrent	1,214us-gaap_InventoryNoncurrent
Total inventories	$ 62,492nwpx_InventoryCurrentAndNonCurrentNet	$ 73,993nwpx_InventoryCurrentAndNonCurrentNet

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Inventory current and non current net.

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Carrying amount, net of valuation reserves and adjustments, as of the balance sheet date of merchandise or goods held by the company that are readily available for sale.

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Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer.

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Inventories not expected to be converted to cash, sold or exchanged within the normal operating cycle.

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Carrying amount, net of valuation reserves and adjustments, as of the balance sheet date of unprocessed items to be consumed in the manufacturing or production process.

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Carrying amount, net of valuation reserves and adjustments, as of the balance sheet date of products used directly or indirectly in the manufacturing or production process, which may or may not become part of the final product. May also include items used in the storage, presentation or transportation of physical goods.

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Carrying amount, net of reserves and adjustments, as of the balance sheet date of merchandise or goods which are partially completed. This inventory is generally comprised of raw materials, labor and factory overhead costs, which require further materials, labor and overhead to be converted into finished goods, and which generally require the use of estimates to determine percentage complete and pricing.

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