Note 8 - Loans - Troubled Debt Restructuring (Details) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
No. of Contracts	15	1	26	3
Pre-Modification Outstanding Recorded Investment	$ 5,127	$ 500	$ 22,678	$ 28,183
Post-Modification Outstanding Recorded Investment	5,127	500	22,678	28,183
Special Reserve
Commercial Portfolio Segment [Member]
No. of Contracts	15	1	18	1
Pre-Modification Outstanding Recorded Investment	$ 5,127	$ 500	$ 7,590	$ 500
Post-Modification Outstanding Recorded Investment	5,127	500	7,590	500
Special Reserve
Commercial Real Estate Portfolio Segment [Member]
No. of Contracts			6
Pre-Modification Outstanding Recorded Investment			$ 14,287
Post-Modification Outstanding Recorded Investment			14,287
Special Reserve	126		$ 126
Real Estate Construction Portfolio Segment [Member]
No. of Contracts				2
Pre-Modification Outstanding Recorded Investment				$ 27,683
Post-Modification Outstanding Recorded Investment				27,683
Special Reserve
Residential Mortgage and Equity Lines Portfolio Segment [Member]
No. of Contracts			2
Pre-Modification Outstanding Recorded Investment			$ 801
Post-Modification Outstanding Recorded Investment			801
Special Reserve	$ 8		$ 8

Amount of allowance for credit losses related to receivables which had been measured under a general allowance for credit losses methodology that are now identified as troubled debt restructurings.

No definition available.

+ Details

Name:	us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=caty_RealEstateConstructionPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=caty_ResidentialMortgageAndEquityLinesPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: