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Note 1 - Summary of Significant Accounting Policies - Estimated Useful Lives of Assets (Details) | 12 Months Ended
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Dec. 31, 2017
Minimum [Member] | Building [Member] |
Assets, estimated useful life (Year) | 15 years
Minimum [Member] | Building Improvements [Member] |
Assets, estimated useful life (Year) | 5 years
Minimum [Member] | Furniture Fixtures and Equipment [Member] |
Assets, estimated useful life (Year) | 3 years
Maximum [Member] | Building [Member] |
Assets, estimated useful life (Year) | 45 years
Maximum [Member] | Building Improvements [Member] |
Assets, estimated useful life (Year) | 20 years
Maximum [Member] | Furniture Fixtures and Equipment [Member] |
Assets, estimated useful life (Year) | 25 years

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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_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |