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Note 3 - Cash and Cash Equivalents (Details Textual) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Average Reserve Balances Required to be Maintained with Federal Bank | $ 7.5 | $ 1.9
Average Excess Balances Maintained With Federal Reserve Bank | 359.5 | $ 338.5
Margin Deposit Assets | $ 4.5 |

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- Definition Amount of average balances maintained with Federal Reserve Bank in excess of the average balances required. + References No definition available. + Details
Name: | caty_AverageExcessBalancesMaintainedWithFederalReserveBank
Namespace Prefix: | caty_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Average reserve balances required to be maintained with federal bank. + References No definition available. + Details
Name: | caty_AverageReserveBalancesRequiredToBeMaintainedWithFederalBank
Namespace Prefix: | caty_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition The amount of cash or securities placed with a broker or counterparty as security for a trading or derivatives securities position which was partially obtained with funds provided by the broker dealer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.8,17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_MarginDepositAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant