Other Real Estate Owned and Property Acquired in Settlement of Loans (Real Estate Owned) (Details) (USD $)	3 Months Ended
Mar. 31, 2015 assets	Mar. 31, 2014
Real Estate Acquired Through Foreclosure [Roll Forward]
Real estate acquired in settlement of loans, beginning of period	$ 20,122,000	$ 28,353,000
Plus: New real estate acquired in settlement of loans	2,495,000	268,000
Less: Sales of real estate acquired in settlement of loans	(1,490,000)	(3,774,000)
Less: Write-downs and net loss on sales charged to expense	(294,000)	(274,000)
Real estate acquired in settlement of loans, end of period	20,833,000	24,573,000
Number of assets under contract for sale	16
Net carrying value of assets under contract for sale	3,400,000
Mortgage loans collateralized by residential real estate properties that are in the process of foreclosure	1,100,000
Real estate - mortgage, 1-4 family residential [Member]
Real Estate Acquired Through Foreclosure [Roll Forward]
Real estate acquired in settlement of loans, end of period	$ 3,400,000

Mortgage Loans in Process of Foreclosure, Amount

No definition available.

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- Definition

This amount represents new real estate acquired in settlement of loans (net of write downs) during the period.

+ References

No definition available.

+ Details

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- Details

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- Definition

Real Estate Acquired Through Foreclosure, Sale Commitment, Carrying Amount

+ References

No definition available.

+ Details

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Namespace Prefix:	cob_
Data Type:	xbrli:monetaryItemType
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- Definition

Real Estate Acquired Through Foreclosure, Sale Commitment, Number of Contracts

+ References

No definition available.

+ Details

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- Definition

This amount represents write-downs on the existing real estate acquired through foreclosure and net gain (loss) on the sales of real estate acquired through foreclosure during the period.

+ References

No definition available.

+ Details

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Namespace Prefix:	cob_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This amount represents sale of real estate acquired in settlement of loans during the period.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4437-111522

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule III

-Article 5

+ Details

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- Details

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