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Residential Mortgage Servicing Rights (Schedule Of Information Relates To Serviced Loan Portfolio) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Transfers and Servicing [Abstract]
Balance of loans serviced for others	$ 8,120,046	$ 8,175,664
MSR as a percentage of serviced loans	1.36%	1.34%

X

- Definition

Balance of loans serviced for others.

+ References

No definition available.

+ Details

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Namespace Prefix:	colb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Mortgage servicing rights, percentage of serviced loans

+ References

No definition available.

+ Details

Name:	colb_MortgageServicingRightsPercentageOfServicedLoans
Namespace Prefix:	colb_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

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- References

No definition available.

+ Details

Name:	us-gaap_TransfersAndServicingAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration