Document And Entity Information	6 Months Ended
Document And Entity Information	Jun. 30, 2011	Apr. 23, 2012
Document And Entity Information
Document Type	10-Q
Amendment Flag	true
Document Period End Date	Jun. 30, 2011
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q2
Entity Registrant Name	NORTHWEST PIPE CO
Entity Central Index Key	0001001385
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		9,371,111
Amendment Description	EXPLANATORY NOTE In this Quarterly Report Amendment No. 1 on Form 10-Q/A for the quarter ended June 30, 2011 (the "June 2011 Form 10-Q/A"), Northwest Pipe Company ("the Company") is restating its previously reported unaudited condensed consolidated financial statements as of June 30, 2011 and December 31, 2010, and for the three and six months ended June 30, 2011 and June 30, 2010 as filed with the Securities and Exchange Commission (the "SEC") on August 8, 2011 (the "Original Filing") in Part I—Item 1, "Financial Statements" to correct material errors identified in such previously issued financial statements. In this filing, the Company is also updating Item 2, "Management's Discussion and Analysis of Financial Condition and Results of Operations" to reflect the effects of the restatement as it relates to June 30, 2011, and the three and six months ended June 30, 2011 and June 30, 2010. Except as required to reflect the effects of the restatement for the items above, no additional modifications or updates to the consolidated financial statements or data in this Form 10-Q/A have been made to the consolidated financial statements or data for the three and six months ended June 30, 2011 or June 30, 2010 as filed in the Original Filing. For further detail on the financial statement impacts and the adjustments made as a result of the restatement, see Note 14 of the Condensed Consolidated Financial Statements in Part I—Item 1, "Financial Statements." Concurrent with the filing of this June 2011 Form 10-Q/A, the Company is filing its Annual Report on Form 10-K for the year ended December 31, 2011 (the "2011 Form 10-K"), which contains restated financial statements as of December 31, 2010 and for the years ended December 31, 2010 and December 31, 2009. The December 31, 2010 condensed consolidated balance sheet included in this June 2011 Form 10-Q/A reflects the restated information included in our 2011 Form 10-K. The Company is also concurrently filing its Quarterly Report on Form 10-Q for the quarter ended September 30, 2011, which includes the restatement of financial statements as of and for the three and nine months ended September 30, 2010 and its amended Quarterly Report on Form 10-Q/A for the quarter ended March 31, 2011 which includes the restatement of our financial statements as of and for the three months ended March 31, 2011 and March 31, 2010. Ineffectiveness of Internal Control over Financial Reporting and Disclosure Controls and Procedures The Company has determined that certain material weaknesses in our internal controls continued to exist as of June 30, 2011. For a description of the material weaknesses in its internal control over financial reporting and its plan to remediate those material weaknesses, see Part I —Item 4, "Controls and Procedures" of this report. In addition, as a result of the existence of material weaknesses in its internal controls, we have also concluded that our disclosure controls and procedures were not effective as of June 30, 2011.