Loans and Allowance for Loan Losses (Unfunded Commitment) (Details) - USD ($) $ in Millions	3 Months Ended
Sep. 30, 2015	Dec. 31, 2014
Unfunded Commitments [Line Items]
Reserve for unfunded commitments	$ 0.8	$ 0.8
Letters of credit [Member]
Unfunded Commitments [Line Items]
Expected usage percentage of remaining unfunded commitments	10.00%

The amount of reserve for unfunded commitments for which the entity is obligated, to lend.

No definition available.

Name:	cob_LoansAndLeasesReceivableUnfundedCommitmentsReserve
Namespace Prefix:	cob_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant