Document and Entity Information	9 Months Ended
Sep. 30, 2011	Oct. 31, 2011
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	true
Amendment Description	Resubmit Interactive Data files for the sole purpose of correcting certain technical and formatting errors.
Document Period End Date	Sep. 30, 2011
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q3
Entity Registrant Name	COLUMBIA BANKING SYSTEM INC
Entity Central Index Key	0000887343
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		39,502,313

Description of changes contained within amended document.

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