Note 6 - Investment Securities (Details) - Investment Securities (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2013	Dec. 31, 2012
Note 6 - Investment Securities (Details) - Investment Securities [Line Items]
Securities Available-for-sale, Gross Unrealized Losses	$ 21,600
Available-for-sale Securities - Fair Value	1,743,309	1,291,480
Total investment securities - Amortized Cost	1,779,859	2,064,444
Total investment securities - Gross Unrealized Gains	16,944	65,092
Total investment securities - Gross Unrealized Losses	53,494	14,150
Total investment securities - Fair Value	1,743,309	2,115,386
Securities Held-to-Maturity, Amortized Cost		773,768
Securities Held-to-Maturity, Fair Value		823,906
US Treasury Securities [Member]
Note 6 - Investment Securities (Details) - Investment Securities [Line Items]
Securities Available-for-sale, Amortized Cost	460,119	509,748
Securities Available-for-sale, Gross Unrealized Gains	214	228
Securities Available-for-sale, Gross Unrealized Losses		5
Available-for-sale Securities - Fair Value	460,333	509,971
Collateralized Mortgage Backed Securities [Member]
Note 6 - Investment Securities (Details) - Investment Securities [Line Items]
Securities Available-for-sale, Amortized Cost	1,131,602	404,505
Securities Available-for-sale, Gross Unrealized Gains	9,364	12,194
Securities Available-for-sale, Gross Unrealized Losses	45,947	5
Available-for-sale Securities - Fair Value	1,095,019	416,694
Securities Held-to-Maturity, Amortized Cost		634,757
Securities Held-to-Maturity, Gross Unrealized Gains		40,801
Securities Held-to-Maturity, Gross Unrealized Losses		0
Securities Held-to-Maturity, Fair Value		675,558
Collateralized Mortgage Obligations [Member]
Note 6 - Investment Securities (Details) - Investment Securities [Line Items]
Securities Available-for-sale, Amortized Cost	6,489	9,772
Securities Available-for-sale, Gross Unrealized Gains	268	430
Securities Available-for-sale, Gross Unrealized Losses	60	34
Available-for-sale Securities - Fair Value	6,697	10,168
Asset-backed Securities [Member]
Note 6 - Investment Securities (Details) - Investment Securities [Line Items]
Securities Available-for-sale, Amortized Cost	127	145
Securities Available-for-sale, Gross Unrealized Gains	1	0
Securities Available-for-sale, Gross Unrealized Losses	0	4
Available-for-sale Securities - Fair Value	128	141
Corporate Debt Securities [Member]
Note 6 - Investment Securities (Details) - Investment Securities [Line Items]
Securities Available-for-sale, Amortized Cost	174,953	349,973
Securities Available-for-sale, Gross Unrealized Gains	186	106
Securities Available-for-sale, Gross Unrealized Losses	7,299	14,102
Available-for-sale Securities - Fair Value	167,840	335,977
Securities Held-to-Maturity, Amortized Cost		9,974
Securities Held-to-Maturity, Gross Unrealized Gains		69
Securities Held-to-Maturity, Gross Unrealized Losses		0
Securities Held-to-Maturity, Fair Value		10,043
Mutual Funds [Member]
Note 6 - Investment Securities (Details) - Investment Securities [Line Items]
Securities Available-for-sale, Amortized Cost	6,000	6,000
Securities Available-for-sale, Gross Unrealized Gains	0	79
Securities Available-for-sale, Gross Unrealized Losses	188	0
Available-for-sale Securities - Fair Value	5,812	6,079
Preferred Stock Of Government Sponsored Entities Member
Note 6 - Investment Securities (Details) - Investment Securities [Line Items]
Securities Available-for-sale, Amortized Cost	569	569
Securities Available-for-sale, Gross Unrealized Gains	6,911	1,766
Securities Available-for-sale, Gross Unrealized Losses	0	0
Available-for-sale Securities - Fair Value	7,480	2,335
Total [Member]
Note 6 - Investment Securities (Details) - Investment Securities [Line Items]
Securities Available-for-sale, Amortized Cost	1,779,859	1,290,676
Securities Available-for-sale, Gross Unrealized Gains	16,944	14,954
Securities Available-for-sale, Gross Unrealized Losses	53,494	14,150
Available-for-sale Securities - Fair Value	1,743,309	1,291,480
Securities Held-to-Maturity, Amortized Cost		773,768
Securities Held-to-Maturity, Gross Unrealized Gains		50,138
Securities Held-to-Maturity, Gross Unrealized Losses		0
Securities Held-to-Maturity, Fair Value		823,906
State and Municipal Securities [Member]
Note 6 - Investment Securities (Details) - Investment Securities [Line Items]
Securities Held-to-Maturity, Amortized Cost		129,037
Securities Held-to-Maturity, Gross Unrealized Gains		9,268
Securities Held-to-Maturity, Gross Unrealized Losses		0
Securities Held-to-Maturity, Fair Value		138,305
Trust Preferred Securities [Member]
Note 6 - Investment Securities (Details) - Investment Securities [Line Items]
Securities Available-for-sale, Amortized Cost		9,964
Securities Available-for-sale, Gross Unrealized Gains		151
Securities Available-for-sale, Gross Unrealized Losses		0
Available-for-sale Securities - Fair Value		$ 10,115

Amortized Cost of Investment Securities

No definition available.

Name:	caty_AmortizedCostOfInvestmentSecurities
Namespace Prefix:	caty_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Gross Unrealized Gains of Investment Securities

+ References

No definition available.

+ Details

Name:	caty_GrossUnrealizedGainsOfInvestmentSecurities
Namespace Prefix:	caty_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Investment securities at fair value.

+ References

No definition available.

+ Details

Name:	caty_InvestmentSecuritiesAtFairValue
Namespace Prefix:	caty_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Investment Securities Gross Unrealized Losses

+ References

No definition available.

+ Details

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

- Details

Name:	caty_Note6InvestmentSecuritiesDetailsInvestmentSecuritiesLineItems
Namespace Prefix:	caty_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

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=28360136&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=27724398&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=6871852&loc=d3e26610-111562

+ Details

Name:	us-gaap_AvailableForSaleSecurities
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

- Definition

Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

+ 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=27724398&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

+ 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=27724398&loc=d3e27161-111563

+ Details

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

- Definition

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.1)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.6)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Fair value of investments in debt securities classified as held-to-maturity.

+ References

+ Details

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

- Definition

Amount of unrecognized gain before loss on investments in held-to-maturity securities.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesUnrecognizedHoldingGains
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of unrecognized loss before gain on investments in held-to-maturity securities. Excludes write-down for other than temporary impairment (OTTI).

+ References

+ Details

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