Allowance for Noncovered Loan and Lease Losses and Unfunded Commitments and Letters of Credit (Narrative) (Details) (Maximum [Member])	6 Months Ended
Jun. 30, 2012
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of unallocated loan amount	5.00%

Unallocated Loan Amount As Percentage Of Allowance

No definition available.

Name:	colb_UnallocatedLoanAmountPercentageOfAllowance
Namespace Prefix:	colb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration