Allowance for Loan and Lease Losses and Unfunded Commitments and Letters of Credit (Changes in the Allowance for Unfunded Commitments and Letters of Credit) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance For Loan And Lease Losses And Unfunded Loan Commitments And Letters Of Credit
Beginning balance	$ 4,330	$ 3,130	$ 2,705
Net changes in the allowance for unfunded commitments and letters of credit	(900)	1,200	425
Ending balance	$ 3,430	$ 4,330	$ 3,130

Allowance for Loan and Lease Losses, Net Changes In Unfunded Commitments and Letters Of Credit Allowance

No definition available.

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