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Note 13 - Commitments and Contingencies (Details Textual) - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Commitments To Extend Notional Amount | $ 2,366,368,000 | $ 2,062,241,000 |
Commitments to Fund Fixed Rate Loans | 99,100,000 |  |
Commitments to Fund Adjustable Rate Loans | 2,300,000,000 |  |
Operating Leases, Rent Expense | 12,000,000 | 10,200,000 | $ 9,300,000
Operating Leases, Income Statement, Sublease Revenue | $ 400,000 | $ 400,000 | $ 300,000
Minimum [Member] |  |  |
Lessor, Operating Lease, Term of Contract | 1 year |  |
Maximum [Member] |  |  |
Lessor, Operating Lease, Term of Contract | 25 years |  |

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- Definition Commitments To Extend Notional Amount + References No definition available. + Details
Name: | caty_CommitmentsToExtendNotionalAmount
Namespace Prefix: | caty_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Commitments to fund adjustable rate loans. + References No definition available. + Details
Name: | caty_CommitmentsToFundAdjustableRateLoans
Namespace Prefix: | caty_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Commitments to fund fixed rate loans. + References No definition available. + Details
Name: | caty_CommitmentsToFundFixedRateLoans
Namespace Prefix: | caty_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Term of the lessor's leasing arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount of revenue recognized for the period under subleasing arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 55 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=77891322&loc=d3e41620-112719 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |