Loans - Schedule of Impaired Loans (Average Recorded Investment in Impaired Loans and Interest Income Recognized) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Impaired Loans: Average recorded investment and interest income
Average recorded investment	$ 33,563	$ 34,290	$ 35,117
Interest income recognized	1,619	2,393	1,796
Interest income that would have been recorded had nonaccrual loans performed	100	200	300
Commercial
Impaired Loans: Average recorded investment and interest income
Average recorded investment	25,280	23,019	16,583
Interest income recognized	1,266	1,764	1,113
Real estate - mortgage
Impaired Loans: Average recorded investment and interest income
Average recorded investment	5,499	6,866	11,164
Interest income recognized	241	312	423
Construction and land
Impaired Loans: Average recorded investment and interest income
Average recorded investment	2,336	2,935	5,957
Interest income recognized	93	104	166
Consumer
Impaired Loans: Average recorded investment and interest income
Average recorded investment	448	1,444	1,362
Interest income recognized	19	202	93
Other
Impaired Loans: Average recorded investment and interest income
Average recorded investment		26	51
Interest income recognized		11	1
Manufacturing \| Commercial
Impaired Loans: Average recorded investment and interest income
Average recorded investment	3,383	5,643	4,244
Interest income recognized	234	306	299
Finance and insurance \| Commercial
Impaired Loans: Average recorded investment and interest income
Average recorded investment	30	164	307
Interest income recognized		24	13
Health care \| Commercial
Impaired Loans: Average recorded investment and interest income
Average recorded investment	194	33	205
Interest income recognized	12	11	13
Real estate services \| Commercial
Impaired Loans: Average recorded investment and interest income
Average recorded investment	6,996	8,006	6,416
Interest income recognized	261	283	281
Construction \| Commercial
Impaired Loans: Average recorded investment and interest income
Average recorded investment	1,873	1,666	1,604
Interest income recognized	140	106	158
Commercial - Other \| Commercial
Impaired Loans: Average recorded investment and interest income
Average recorded investment	12,804	7,507	3,807
Interest income recognized	619	1,034	349
Residential and commercial owner-occupied \| Real estate - mortgage
Impaired Loans: Average recorded investment and interest income
Average recorded investment	1,777	1,762	4,102
Interest income recognized	114	130	176
Residential and commercial investor \| Real estate - mortgage
Impaired Loans: Average recorded investment and interest income
Average recorded investment	3,722	5,104	7,062
Interest income recognized	$ 127	$ 182	$ 247

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5258-111524

+ Details

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

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18794-111554

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis=cobz_ManufacturingFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis=cobz_FinanceAndInsuranceFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis=cobz_HealthCareFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis=cobz_RealEstateServicesFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis=cobz_ConstructionFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis=cobz_OtherFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=cobz_ResidentialAndCommercialOwnerOccupiedFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=cobz_ResidentialAndCommercialInvestorFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: