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Allowance for Credit Losses (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Financing Receivable, Allowance for Credit Loss [Line Items]
Total allowance for credit losses	$ 438,983	$ 438,599	$ 438,983	$ 438,599	$ 440,800
Change in Allowance for Credit Losses			$ (1,800)
Portion of ACL Driven by Modeled Results	83.00%		83.00%
Financing Receivable, Excluding Accrued Interest, Nonaccrual, No Allowance	$ 2,800		$ 2,800		3,600
Interest Income on Non-Accrual Loans and Leases	0	$ 0	0	$ 0
Collateral Pledged
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing Receivable, Excluding Accrued Interest, Nonaccrual, No Allowance	$ 46,600		$ 46,600		$ 59,400

X

- Definition

Financing Receivable, Allowance for Credit Loss and Unfunded Lending Commitments, Excluding interest, Change

+ References

No definition available.

+ Details

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Balance Type:	credit
Period Type:	duration

X

- Definition

Financing Receivable, Allowance for Credit Loss and Unfunded Lending Commitments, Excluding Accrued Interest, Total

+ References

No definition available.

+ Details

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

X

- Definition

Portion of ACL Driven by Modeled Results

+ References

No definition available.

+ Details

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Namespace Prefix:	colb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph c
-Publisher SEC

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

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X

- Definition

Amortized cost excluding accrued interest, of financing receivable on nonaccrual status with no allowance for credit loss. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-16

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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X

- Definition

Amount of interest income on financing receivable on nonaccrual status.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-16

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Period Type:	duration

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