Allowance for Loan and Lease Losses and Unfunded Commitments and Letters of Credit (Allowance for Loan and Lease Losses) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 71,021	$ 69,264	$ 70,043	$ 68,172
Charged-offs	(6,018)	(6,234)	(9,833)	(13,139)
Recoveries	4,804	2,634	6,822	5,377
Provision for loan and lease losses	3,177	3,640	5,952	8,894
Ending balance	72,984	69,304	72,984	69,304
Specific Reserve	3,499	2,705	3,499	2,705
General Allocation	69,485	66,599	69,485	66,599
Secured Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	35,672	32,114	36,050	32,321
Charged-offs	(3,600)	(2,900)	(4,709)	(6,670)
Recoveries	2,903	728	3,200	1,339
Provision for loan and lease losses	4,564	1,866	4,998	4,818
Ending balance	39,539	31,808	39,539	31,808
Specific Reserve	3,425	2,486	3,425	2,486
General Allocation	36,114	29,322	36,114	29,322
Unsecured Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,188	1,300	960	1,299
Charged-offs	0	(41)	(18)	(44)
Recoveries	41	25	109	76
Provision for loan and lease losses	(82)	(19)	96	(66)
Ending balance	1,147	1,265	1,147	1,265
Specific Reserve	0	0	0	0
General Allocation	1,147	1,265	1,147	1,265
One-to-Four Family Residential [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	645	654	599	916
Charged-offs	(153)	(35)	(460)	(35)
Recoveries	223	20	340	61
Provision for loan and lease losses	(87)	35	149	(268)
Ending balance	628	674	628	674
Specific Reserve	4	1	4	1
General Allocation	624	673	624	673
Commercial Land [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	2,288	1,262	1,797	1,178
Charged-offs	0	(26)	0	(26)
Recoveries	0	2	0	2
Provision for loan and lease losses	68	184	559	268
Ending balance	2,356	1,422	2,356	1,422
Specific Reserve	0	0	0	0
General Allocation	2,356	1,422	2,356	1,422
Income Property Multifamily [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	6,803	7,402	7,342	6,616
Charged-offs	0	0	0	0
Recoveries	60	120	95	181
Provision for loan and lease losses	(9)	524	(583)	1,249
Ending balance	6,854	8,046	6,854	8,046
Specific Reserve	25	100	25	100
General Allocation	6,829	7,946	6,829	7,946
Owner Occupied [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	6,534	6,086	6,439	5,550
Charged-offs	0	0	0	0
Recoveries	67	8	110	16
Provision for loan and lease losses	(89)	242	(37)	770
Ending balance	6,512	6,336	6,512	6,336
Specific Reserve	0	0	0	0
General Allocation	6,512	6,336	6,512	6,336
Land And Acquisition [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	509	640	316	339
Charged-offs	0	0	(14)	0
Recoveries	27	2	47	53
Provision for loan and lease losses	(175)	(55)	12	195
Ending balance	361	587	361	587
Specific Reserve	0	0	0	0
General Allocation	361	587	361	587
Residential Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,109	1,449	669	733
Charged-offs	0	0	0	0
Recoveries	31	3	40	206
Provision for loan and lease losses	237	(76)	668	437
Ending balance	1,377	1,376	1,377	1,376
Specific Reserve	0	0	0	0
General Allocation	1,377	1,376	1,377	1,376
Income Property Multifamily Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	782	715	404	388
Charged-offs	0	0	0	0
Recoveries	0	1	0	2
Provision for loan and lease losses	203	188	581	514
Ending balance	985	904	985	904
Specific Reserve	0	0	0	0
General Allocation	985	904	985	904
Owner Occupied Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,768	1,210	1,192	1,006
Charged-offs	0	0	0	0
Recoveries	0	0	0	0
Provision for loan and lease losses	(386)	174	190	378
Ending balance	1,382	1,384	1,382	1,384
Specific Reserve	0	0	0	0
General Allocation	1,382	1,384	1,382	1,384
Consumer Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	3,360	3,368	3,534	3,531
Charged-offs	(465)	(334)	(893)	(600)
Recoveries	248	201	533	366
Provision for loan and lease losses	408	325	377	263
Ending balance	3,551	3,560	3,551	3,560
Specific Reserve	45	118	45	118
General Allocation	3,506	3,442	3,506	3,442
Purchased Credit Impaired Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	9,395	13,064	10,515	13,726
Charged-offs	(1,800)	(2,898)	(3,739)	(5,764)
Recoveries	1,204	1,524	2,348	3,075
Provision for loan and lease losses	(738)	91	(1,063)	744
Ending balance	8,061	11,781	8,061	11,781
Specific Reserve	0	0	0	0
General Allocation	8,061	11,781	8,061	11,781
Unallocated Financing Receivables [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	968	0	226	569
Charged-offs	0	0	0	0
Recoveries	0	0	0	0
Provision for loan and lease losses	(737)	161	5	(408)
Ending balance	231	161	231	161
Specific Reserve	0	0	0	0
General Allocation	$ 231	$ 161	$ 231	$ 161

Loan and Lease Losses General Allocation

No definition available.

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

- Definition

Loan and Lease Losses Specific Reserve

+ References

No definition available.

+ Details

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

- Definition

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

+ References

+ Details

Name:	us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

+ Details

Name:	us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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 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 2: 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=98514028&loc=d3e3602-108585

Reference 3: 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=84173941&loc=SL6953423-111524

+ Details

Name:	us-gaap_ProvisionForLoanAndLeaseLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_SecuredLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_UnsecuredLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_OneToFourFamilyResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_CommercialLandMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_IncomePropertyMultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_OwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_LandAndAcquisitionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_ResidentialConstructionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_IncomePropertyMultifamilyConstructionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_OwnerOccupiedConstructionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_PurchasedCreditImpairedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_UnallocatedFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: