Inventories - Schedule of Inventories (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Inventory Disclosure [Abstract]
Raw materials	$ 6,289	$ 5,383
Work in process	2,488us-gaap_InventoryWorkInProcess	2,090us-gaap_InventoryWorkInProcess
Finished goods	10,791us-gaap_InventoryFinishedGoods	9,548us-gaap_InventoryFinishedGoods
Allowance for obsolescence	(581)nrthf_AllowanceForObsolescence	(565)nrthf_AllowanceForObsolescence
Total inventory - net	$ 18,987us-gaap_InventoryNet	$ 16,456us-gaap_InventoryNet

Allowance For Obsolescence

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- Definition

Amount before valuation and LIFO reserves of completed merchandise or goods expected to be sold within one year or operating cycle, if longer.

+ References

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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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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-Publisher FASB

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Gross amount of unprocessed materials to be used in manufacturing or production process and supplies that will be consumed.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

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Amount before valuation and LIFO reserves of merchandise or goods in the production process expected to be completed within one year or operating cycle, if longer.

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