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Note 9 - Borrowed Funds (Details) (USD $)
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Note 9 - Borrowed Funds (Details) [Line Items]      
Securities Sold under Agreements to Repurchase 400,000,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase $ 450,000,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase  
Weighted Average Rate of Securities Sold Under Agreements 3.89%caty_WeightedAverageRateOfSecuritiesSoldUnderAgreements 3.85%caty_WeightedAverageRateOfSecuritiesSoldUnderAgreements  
Prepaid Security Sold Under Repurchase Agreement     100,000,000caty_PrepaidSecuritySoldUnderRepurchaseAgreement
Rate of Prepaid Security Sold Under Repurchase Agreements     3.50%caty_RateOfPrepaidSecuritySoldUnderRepurchaseAgreements
Prepayment Penalty Security Sold Under Agreements to Repurchase     3,400,000caty_PrepaymentPenaltySecuritySoldUnderAgreementsToRepurchase
Federal Home Loan Bank, Advances, Maturities Summary, Due in Next Twelve Months 460,000,000us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueWithinOneYearOfBalanceSheetDate 400,000,000us-gaap_FederalHomeLoanBankAdvancesMaturitiesSummaryDueWithinOneYearOfBalanceSheetDate  
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate 0.24%us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankInterestRate 0.27%us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankInterestRate  
Long-term Federal Home Loan Bank Advances 25,000,000us-gaap_FederalHomeLoanBankAdvancesLongTerm 25,000,000us-gaap_FederalHomeLoanBankAdvancesLongTerm  
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Weighted Average Interest Rate 1.13%us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankWeightedAverageInterestRate 1.13%us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankWeightedAverageInterestRate  
Callable Securities [Member]      
Note 9 - Borrowed Funds (Details) [Line Items]      
Number of Floating-to-fixed Rate Agreements 4caty_NumberOfFloatingToFixedRateAgreements
/ us-gaap_FinancialInstrumentAxis
= caty_CallableSecuritiesMember		    
Non Callable Fixed Rate Securities [Member]      
Note 9 - Borrowed Funds (Details) [Line Items]      
Securities Sold under Agreements to Repurchase 200,000,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
/ us-gaap_FinancialInstrumentAxis
= caty_NonCallableFixedRateSecuritiesMember		    
Weighted Average Rate of Securities Sold Under Agreements 2.78%caty_WeightedAverageRateOfSecuritiesSoldUnderAgreements
/ us-gaap_FinancialInstrumentAxis
= caty_NonCallableFixedRateSecuritiesMember		    
Mature in August 2016 [Member]      
Note 9 - Borrowed Funds (Details) [Line Items]      
Long-term Debt, Maturities, Repayments of Principal in Year Two 50,000,000us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= caty_MatureInAugust2016Member		    
Mature in July 2017 [Member]      
Note 9 - Borrowed Funds (Details) [Line Items]      
Long-term Debt, Maturities, Repayments of Principal in Year Three 50,000,000us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= caty_MatureInJuly2017Member		    
Mature in June 2018 [Member]      
Note 9 - Borrowed Funds (Details) [Line Items]      
Long-term Debt, Maturities, Repayments of Principal in Year Four 50,000,000us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= caty_MatureInJune2018Member		    
Mature in July 2018 [Member]      
Note 9 - Borrowed Funds (Details) [Line Items]      
Long-term Debt, Maturities, Repayments of Principal in Year Five 50,000,000us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive
/ us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis
= caty_MatureInJuly2018Member		    
Floating to Fixed Rate Agreements Totaling $200 Million [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]      
Note 9 - Borrowed Funds (Details) [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 3.40%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ caty_FinancialInstrumentsOwnedAxis
= caty_FloatingToFixedRateAgreementsTotaling200MillionMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember		    
Floating to Fixed Rate Agreements Totaling $200 Million [Member] | Maximum [Member]      
Note 9 - Borrowed Funds (Details) [Line Items]      
Debt Instrument, Interest Rate, Stated Percentage 5.07%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ caty_FinancialInstrumentsOwnedAxis
= caty_FloatingToFixedRateAgreementsTotaling200MillionMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		    
Floating to Fixed Rate Agreements Totaling $200 Million [Member] | Minimum [Member]      
Note 9 - Borrowed Funds (Details) [Line Items]      
Debt Instrument, Interest Rate, Stated Percentage 4.89%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ caty_FinancialInstrumentsOwnedAxis
= caty_FloatingToFixedRateAgreementsTotaling200MillionMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Floating to Fixed Rate Agreements Totaling $200 Million [Member]      
Note 9 - Borrowed Funds (Details) [Line Items]      
Number of Floating-to-fixed Rate Agreements 4caty_NumberOfFloatingToFixedRateAgreements
/ caty_FinancialInstrumentsOwnedAxis
= caty_FloatingToFixedRateAgreementsTotaling200MillionMember		    
Total Floating to Fixed Rate 200,000,000caty_TotalFloatingToFixedRate
/ caty_FinancialInstrumentsOwnedAxis
= caty_FloatingToFixedRateAgreementsTotaling200MillionMember		    
Debt, Weighted Average Interest Rate 5.00%us-gaap_DebtWeightedAverageInterestRate
/ caty_FinancialInstrumentsOwnedAxis
= caty_FloatingToFixedRateAgreementsTotaling200MillionMember		    
Floating Interest Rate, Range of Time Period 1 year    
Securities Sold under Agreements to Repurchase [Member]      
Note 9 - Borrowed Funds (Details) [Line Items]      
Investment Securities Pledged As Collateral 467,500,000caty_InvestmentSecuritiesPledgedAsCollateral
/ caty_FinancialInstrumentsOwnedAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember		 $ 516,300,000caty_InvestmentSecuritiesPledgedAsCollateral
/ caty_FinancialInstrumentsOwnedAxis
= us-gaap_SecuritiesSoldUnderAgreementsToRepurchaseMember