Loans and Allowance for Loan Losses (Loans Pledged) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Loans Receivable, Net [Abstract]
Loans pledged to collateralize FHLB advances and letters of credit	$ 121.4	$ 127.2
Pledged financial instruments, Not separately reported, securities, for Federal Home Loan Bank debt	123.1	124.6
Credit availability for borrowing under FHLB advances and letters of credit	126.2	130.8
Loans pledged to collateralize potential borrowings from Federal Reserve Discount Window	5.3
Pledged Financial Instruments, Not Separately Reported, investment securities, for Federal Reserve Bank	47.8
Borrowing capacity under Federal Reserve Discount Window	51.7
Federal Home Loan Bank Advances and Line of Credit, Amount of Available Borrowing	$ 246.0

Amount of available for borrowing from FHLB advance and line of credit as of balance sheet date.

No definition available.

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- Definition

Borrowing capacity with Federal Reserve Discount Window as of balance sheet date.

+ References

No definition available.

+ Details

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- Definition

Pledged Financial Instruments, Not Separately Reported, investment securities, for Federal Reserve Bank

+ References

No definition available.

+ Details

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- Definition

The carrying amount, as of the date of the latest financial statement presented, of loans receivable which are owned but transferred to serve as collateral for the payment of Federal Reserve debt obligations, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

+ References

No definition available.

+ Details

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- Definition

Pledged financial instruments, Not separately reported, securities, for Federal Home Loan Bank debt

+ References

No definition available.

+ Details

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Namespace Prefix:	cob_
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- Definition

Amount of advances or credit lines available from the Federal Home Loan Bank (FHLB).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 470

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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 210.9-03.13(3)(a))

-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 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22 (b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 13

-Subparagraph 3(a)

-Article 9

+ Details

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- Definition

The carrying amount, as of the date of the latest financial statement presented, of loans receivable which are owned but transferred to serve as collateral for the payment of FHLB debt obligations, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 30

-Section 50

-Paragraph 1A

-Subparagraph (b)(1)(i)

-URI http://asc.fasb.org/extlink&oid=7523297&loc=SL6224234-111729

+ Details

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