Note 6 - Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
US Treasury Securities [Member]
Securities Available-for-Sale, Amortized Cost	$ 239,811	$ 284,678
Securities Available-for-Sale, Gross Unrealized Gains	228	5
Securities Available-for-Sale, Gross Unrealized Losses		395
Securities available-for-sale (amortized cost of $1,227,169 in 2016 and $1,595,723 in 2015	240,039	284,288
US Government-sponsored Enterprises Debt Securities [Member]
Securities Available-for-Sale, Amortized Cost	125,000	150,000
Securities Available-for-Sale, Gross Unrealized Gains	142
Securities Available-for-Sale, Gross Unrealized Losses		1,840
Securities available-for-sale (amortized cost of $1,227,169 in 2016 and $1,595,723 in 2015	125,142	148,160
Collateralized Mortgage Backed Securities [Member]
Securities Available-for-Sale, Amortized Cost	774,629	1,073,108
Securities Available-for-Sale, Gross Unrealized Gains	8,690	560
Securities Available-for-Sale, Gross Unrealized Losses	6	11,399
Securities available-for-sale (amortized cost of $1,227,169 in 2016 and $1,595,723 in 2015	783,313	1,062,269
Collateralized Mortgage Obligations [Member]
Securities Available-for-Sale, Amortized Cost	56	63
Securities Available-for-Sale, Gross Unrealized Gains
Securities Available-for-Sale, Gross Unrealized Losses	24	27
Securities available-for-sale (amortized cost of $1,227,169 in 2016 and $1,595,723 in 2015	32	36
Corporate Debt Securities [Member]
Securities Available-for-Sale, Amortized Cost	74,960	74,955
Securities Available-for-Sale, Gross Unrealized Gains	48	425
Securities Available-for-Sale, Gross Unrealized Losses	1,779	1,525
Securities available-for-sale (amortized cost of $1,227,169 in 2016 and $1,595,723 in 2015	73,229	73,855
Mutual Funds [Member]
Securities Available-for-Sale, Amortized Cost	6,000	6,000
Securities Available-for-Sale, Gross Unrealized Gains
Securities Available-for-Sale, Gross Unrealized Losses	47	167
Securities available-for-sale (amortized cost of $1,227,169 in 2016 and $1,595,723 in 2015	5,953	5,833
Preferred Stock of Government Sponsored Entities [Member]
Securities Available-for-Sale, Amortized Cost	2,811	2,811
Securities Available-for-Sale, Gross Unrealized Gains	1,349	633
Securities Available-for-Sale, Gross Unrealized Losses		228
Securities available-for-sale (amortized cost of $1,227,169 in 2016 and $1,595,723 in 2015	4,160	3,216
Other Equity Securities [Member]
Securities Available-for-Sale, Amortized Cost	3,902	4,108
Securities Available-for-Sale, Gross Unrealized Gains	6,134	4,929
Securities Available-for-Sale, Gross Unrealized Losses		342
Securities available-for-sale (amortized cost of $1,227,169 in 2016 and $1,595,723 in 2015	10,036	8,695
Securities Available-for-Sale, Amortized Cost	1,227,169	1,595,723
Securities Available-for-Sale, Gross Unrealized Gains	16,591	6,552
Securities Available-for-Sale, Gross Unrealized Losses	1,856	15,923
Securities available-for-sale (amortized cost of $1,227,169 in 2016 and $1,595,723 in 2015	$ 1,241,904	$ 1,586,352

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

+ Details

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

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

+ Details

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

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=caty_MutualFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=caty_PreferredStockOfGovernmentSponsoredEntitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=caty_OtherEquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: