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Investment Securities (Maturity Schedule) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Amortized Cost    
Total, amortized cost $ 370,038us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCost  
Estimated Fair Value    
Estimated fair value 367,842us-gaap_AvailableForSaleSecurities 350,040us-gaap_AvailableForSaleSecurities [1]
Amortized Cost    
Total, amortized cost 140,559us-gaap_HeldToMaturitySecurities 142,461us-gaap_HeldToMaturitySecurities [1]
Estimated Fair Value    
Estimated fair value 140,959us-gaap_HeldToMaturitySecuritiesFairValue 140,875us-gaap_HeldToMaturitySecuritiesFairValue [1]
US Government-sponsored Enterprises Debt Securities [Member]    
Amortized Cost    
Due after one year through five years, amortized cost 2,022us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		  
Estimated Fair Value    
Due after one year through five years, estimated fair value 2,036us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		  
Estimated fair value 2,036us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		 2,044us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Residential Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]    
Amortized Cost    
Due after five years through 10 years 2,200us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Due after ten years 286,422us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Estimated Fair Value    
Due after five years through 10 years 2,263us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Due after ten years 283,456us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Estimated fair value 285,719us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 290,145us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Amortized Cost    
Due after ten years 130,496us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Estimated Fair Value    
Due after ten years 130,714us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Estimated fair value 130,714us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 130,877us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Residential Mortgage-backed Securities, Issued by Private Enterprises [Member]    
Amortized Cost    
Due after ten years 15,744us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Estimated Fair Value    
Due after ten years 16,430us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Estimated fair value 16,430us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 17,271us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_ResidentialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Commercial mortgage-backed securities - GSE [Member]    
Amortized Cost    
Due after five years through 10 years 22,279us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesGseMember		  
Estimated Fair Value    
Due after five years through 10 years 22,208us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesGseMember		  
Estimated fair value 22,208us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesGseMember		 21,958us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesGseMember
Commercial Mortgage-backed Securities, Issued by Private Enterprises [Member]    
Amortized Cost    
Due after ten years 10,354us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Estimated Fair Value    
Due after ten years 10,457us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Estimated fair value 10,457us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 10,215us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Amortized Cost    
Due after ten years 10,063us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsNetCarryingAmount
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Estimated Fair Value    
Due after ten years 10,245us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Estimated fair value 10,245us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 9,998us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cob_CommercialMortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Corporate notes [Member]    
Amortized Cost    
Due after one year through five years, amortized cost 31,017us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateNoteSecuritiesMember		  
Estimated Fair Value    
Due after one year through five years, estimated fair value 30,992us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateNoteSecuritiesMember		  
Estimated fair value $ 30,992us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateNoteSecuritiesMember		 $ 8,407us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateNoteSecuritiesMember
[1] Derived from audited consolidated financial statements