Note 7 - Loans - Allowance for Loan Losses by Portfolio Segment and Based on Impairment Method (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Loans individually evaluated for impairment, balance	$ 143,863	$ 115,074
Loans collectively evaluated for impairment, allowance	114,248	116,170
Loans collectively evaluated for impairment, balance	11,427,369	11,086,201
Loans, allowance	115,809	118,966
Loans	11,571,232	11,201,275
Loans individually evaluated for impairment, allowance	1,561	2,796
Commercial Portfolio Segment [Member]
Loans individually evaluated for impairment, balance	18,858	27,761
Loans collectively evaluated for impairment, allowance	46,599	47,376
Loans collectively evaluated for impairment, balance	2,197,102	2,220,426
Loans, allowance	46,744	49,203
Loans	2,215,960	2,248,187
Loans individually evaluated for impairment, allowance	145	1,827
Real Estate Construction Portfolio Segment [Member]
Loans individually evaluated for impairment, balance	45,057	5,458
Loans collectively evaluated for impairment, allowance	17,844	23,268
Loans collectively evaluated for impairment, balance	502,680	542,630
Loans, allowance	17,844	23,268
Loans	547,737	548,088
Loans individually evaluated for impairment, allowance
Commercial Real Estate Portfolio Segment [Member]
Loans individually evaluated for impairment, balance	65,313	64,470
Loans collectively evaluated for impairment, allowance	35,822	34,291
Loans collectively evaluated for impairment, balance	5,818,457	5,720,778
Loans, allowance	36,840	34,864
Loans	5,883,770	5,785,248
Loans individually evaluated for impairment, allowance	1,018	573
Residential Mortgage and Equity Lines Portfolio Segment [Member]
Loans individually evaluated for impairment, balance	14,635	17,385
Loans collectively evaluated for impairment, allowance	13,966	11,224
Loans collectively evaluated for impairment, balance	2,903,573	2,598,374
Loans, allowance	14,364	11,620
Loans	2,918,208	2,615,759
Loans individually evaluated for impairment, allowance	398	396
Consumer Portfolio Segment [Member]
Loans individually evaluated for impairment, balance
Loans collectively evaluated for impairment, allowance	17	11
Loans collectively evaluated for impairment, balance	5,557	3,993
Loans, allowance	17	11
Loans	5,557	3,993
Loans individually evaluated for impairment, allowance

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
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=us-gaap_CommercialRealEstatePortfolioSegmentMember
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:

- Details

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