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Note 1 - Summary of Significant Accounting Policies - Estimated Useful Lives and Methods of Depreciation (Details) | 9 Months Ended
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Sep. 30, 2018
Banking Premises [Member] |
Property, plant and equipment, depreciation method | Straight-Line and Accelerated
Banking Premises [Member] | Minimum [Member] |
Property, plant and equipment, useful life (Year) | 15 years
Banking Premises [Member] | Maximum [Member] |
Property, plant and equipment, useful life (Year) | 40 years
Furniture and Equipment [Member] |
Property, plant and equipment, depreciation method | Straight-Line and Accelerated
Furniture and Equipment [Member] | Minimum [Member] |
Property, plant and equipment, useful life (Year) | 5 years
Furniture and Equipment [Member] | Maximum [Member] |
Property, plant and equipment, useful life (Year) | 10 years

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- Definition Description of the methodology for computing depreciation for classes of depreciable assets. + References No definition available. + Details
Name: | cban_PropertyPlantAndEquipmentDepreciationMethod
Namespace Prefix: | cban_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=cban_BankingPremisesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=cban_FurnitureAndEquipmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |