Other Real Estate Owned and Property Acquired in Settlement of Loans (Changes in Real Estate Acquired in Settlement) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Real Estate Acquired Through Foreclosure [Roll Forward]
Real estate acquired in settlement of loans, beginning of period	$ 16,251	$ 20,122
Plus: New real estate acquired in settlement of loans	59	2,495
Less: Sales of real estate acquired in settlement of loans	(1,964)	(1,490)
Less: Write-downs and net loss on sales charged to expense	(501)	(294)
Real estate acquired in settlement of loans, end of period	$ 13,845	$ 20,833

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

No definition available.

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Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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

Name:	cob_SaleOfRealEstateAcquiredThroughForeclosure
Namespace Prefix:	cob_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5162-111524

Reference 2: 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 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 28
-Article 12

Reference 4: 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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant