Basis of Presentation and Significant Accounting Policies Correction of immaterial error (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Sep. 30, 2014
Correction of immaterial error, pre-tax increase to interest income on taxable securities	$ 2.6

Correction of immaterial error, pre-tax increase of interest income on taxable securities

No definition available.

Name:	colb_Correctionofimmaterialerrorpretaxincreasetointerestincomeontaxablesecurities
Namespace Prefix:	colb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration