v2.4.1.9

Allowance for Loan and Lease Losses and Unfunded Commitments and Letters of Credit (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2015 | Mar. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Provision for loan and lease losses | $ 1,209 us-gaap_ProvisionForLoanAndLeaseLosses | [1] | $ 1,922 us-gaap_ProvisionForLoanAndLeaseLosses | [1]
Maximum [Member] |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Percentage of unallocated loan amount | 5.00%colb_UnallocatedLoanAmountPercentageOfAllowance / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |
[1] | {F|ahBzfndlYmZpbGluZ3MtaHJkcmoLEgZYTUxEb2MiXlhCUkxEb2NHZW5JbmZvOjNlZWQyMzNmYzU4NTQ3YjE4YmZiNDE3NjBhNjc1ZWQ4fFRleHRTZWxlY3Rpb246QTFEODJCNjcwRTEwQ0JFQ0Q1OEYwNERBMjg2MDZBQzAM}

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- Definition Unallocated Loan Amount As Percentage Of Allowance + References No definition available. + Details
Name: | colb_UnallocatedLoanAmountPercentageOfAllowance
Namespace Prefix: | colb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of expense related to estimated loss from loan and lease transactions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 11 -Article 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.11) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524 + Details
Name: | us-gaap_ProvisionForLoanAndLeaseLosses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |