Note 8 - Loans (Details Textual) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Dec. 31, 2020
Impaired Financing Receivable, Recorded Investment, Total	$ 94,446	$ 95,405
Percentage Of Charge Off To Contractual Balances For Impaired Loans	15.10%	7.10%
Financing Receivable, Troubled Debt Restructuring, Subsequent Default	$ 0
Financing Receivable, Troubled Debt Restructuring, Commitment to Lend	0
Loans and Leases Receivable, Gross, Total	15,651,848	$ 15,644,396
Loans and Leases Receivable, Allowance, Ending Balance	147,264	166,538
Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable, Gross, Total	$ 7,549,522	7,555,027
Loans and Leases Receivable, Allowance, Ending Balance		49,205
Financing Receivable, Number of Contracts with Modifications	7
Loans and Leases Receivable with Modifications, Gross	$ 56,100
Financing Receivable with Modifications, Percentage of Total Commercial Real Estate Loans	0.70%
Commercial Portfolio Segment [Member]
Loans and Leases Receivable, Gross, Total	$ 2,890,693	2,836,833
Loans and Leases Receivable, Allowance, Ending Balance		68,742
Financing Receivable, Number of Contracts with Modifications	6
Loans and Leases Receivable with Modifications, Gross	$ 38,000
Financing Receivable with Modifications, Percentage of Total Commercial Real Estate Loans	1.30%
Residential Portfolio Segment [Member]
Loans and Leases Receivable, Gross, Total	$ 4,102,203	4,145,389
Financing Receivable with Modifications, Percentage of Total Commercial Real Estate Loans	0.80%
Financing Receivable, Number of Contracts with Approved Forbearance Requests	52
Loans and Leases Receivable with Approved Forbearance Requests, Gross	$ 31,400
HELOC Loans [Member]
Financing Receivable with Modifications, Percentage of Total Commercial Real Estate Loans	0.30%
Financing Receivable, Number of Contracts with Approved Forbearance Requests	2
Loans and Leases Receivable with Approved Forbearance Requests, Gross	$ 1,100
SBA's Paycheck Protection Program [Member]
Financing Receivable, Number of Contracts	2,770
Loans and Leases Receivable, Gross, Total	$ 334,400
Loans and Leases Receivable, Forgiven	71,900
Loans and Leases Receivable, Allowance, Ending Balance	$ 0
Minimum [Member]
Period Loan Is In Payment Default (Day)	60 days
Maximum [Member]
Period Loan Is In Payment Default (Day)	90 days
Nonaccrual Loans [Member]
Impaired Financing Receivable, Recorded Investment, Total		67,700
Accruing Troubled Debt Restructuring [Member]
Impaired Financing Receivable, Recorded Investment, Total		27,700
Financing Receivable, Troubled Debt Restructuring	$ 27,900	27,700
Troubled Debt Restructuring, Specific Reserves	56	122
Non-accruing Troubled Debt Restructuring [Member]
Financing Receivable, Troubled Debt Restructuring	8,600	9,000
Troubled Debt Restructuring, Specific Reserves	$ 1	$ 24

The number of financing receivables contracts.

No definition available.

Name:	caty_FinancingReceivableNumberOfContracts
Namespace Prefix:	caty_
Data Type:	xbrli:integerItemType
Balance Type:	na
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- Definition

The number of financing receivable contracts with approved forbearance requests.

+ References

No definition available.

+ Details

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- Definition

The number of financing receivables contracts with modifications.

+ References

No definition available.

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- Definition

The percentage of financing receivable with modifications to total commercial real estate loans.

+ References

No definition available.

+ Details

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Namespace Prefix:	caty_
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- Definition

With regard to loans and leases receivable, this element represents the amount that has been forgiven.

+ References

No definition available.

+ Details

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Namespace Prefix:	caty_
Data Type:	xbrli:monetaryItemType
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- Definition

Amount before allowance of loans and leases held in portfolio with approved forbearance requests, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

No definition available.

+ Details

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- Definition

Amount before allowance of loans and leases held in portfolio with modifications, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

No definition available.

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Namespace Prefix:	caty_
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Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	caty_
Data Type:	num:percentItemType
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- Definition

Period Loan Is In Payment Default

+ References

No definition available.

+ Details

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Namespace Prefix:	caty_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the amount of specific reserves associated with troubled debt restructurings.

+ References

No definition available.

+ Details

Name:	caty_TroubledDebtRestructuringSpecificReserves
Namespace Prefix:	caty_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953676-111524

Reference 2: 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=121585226&loc=d3e18845-111554

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

- Definition

Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953682-111524

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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=121611835&loc=SL6953550-111524

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

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
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-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

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

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4428-111522

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

- Definition

Amount of unfunded portion of contractual agreement to lend funds to borrower for financing receivable modified as troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18794-111554

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Namespace Prefix:	us-gaap_
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Period Type:	instant

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- Details

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- Details

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