Percentage of charge-off to contractual balances for impaired loans.

No definition available.

Name:	caty_PercentageOfChargeOffToContractualBalancesForImpairedLoans
Namespace Prefix:	caty_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Period Loan Is In Payment Default

+ References

No definition available.

+ Details

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- Definition

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.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of the outstanding recorded investment related to 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 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953682-111524

+ Details

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

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Reflects the unfunded portion of contractual agreements to lend funds to borrowers for loans whose terms have been modified by troubled debt restructuring.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18794-111554

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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- Details

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- Details

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Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_ClassOfFinancingReceivableTypeOfBorrowerAxis=caty_AccruingTroubledDebtRestructuringMember
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- Details

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Data Type:	na
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- Details

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Note 8 - Loans (Details Textual) - USD ($)	3 Months Ended	12 Months Ended
Note 8 - Loans (Details Textual) - USD ($)	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Impaired Financing Receivable, Recorded Investment, Total	$ 132,113,000		$ 117,353,000
Percentage Of Charge Off To Contractual Balances For Impaired Loans	6.40%		7.20%
Allowance for Credit Losses, Change in Method of Calculating Impairment	$ 142,000
Financing Receivable, Modifications, Subsequent Default, Recorded Investment	0
Loans and Leases Receivable, Impaired, Commitment to Lend	0
Minimum [Member]
Period Loan Is In Payment Default		60 days
Maximum [Member]
Period Loan Is In Payment Default		90 days
Nonaccrual Loans [Member]
Impaired Financing Receivable, Recorded Investment, Total			$ 48,800,000
Accruing Troubled Debt Restructuring [Member]
Impaired Financing Receivable, Recorded Investment, Total			68,600,000
Financing Receivable, Modifications, Recorded Investment	82,800,000		68,600,000
Allowance for Credit Losses, Change in Method of Calculating Impairment	1,200,000		1,900,000
Non-accruing Troubled Debt Restructuring [Member]
Financing Receivable, Modifications, Recorded Investment	31,200,000		33,400,000
Allowance for Credit Losses, Change in Method of Calculating Impairment	75,000		83,000
Nonaccrual Loans [Member]
Impaired Financing Receivable, Recorded Investment, Total	49,300,000
Accruing Troubled Debt Restructuring [Member]
Impaired Financing Receivable, Recorded Investment, Total	82,800,000
Financing Receivable, Modifications, Recorded Investment	$ 82,785,000		$ 68,565,000	$ 80,419,000	$ 65,393,000