Note 2 - Business Combinations (Details Textual) - SinoPac Bancorp [Member] $ in Millions	Jul. 14, 2017 USD ($) shares
Business Combination, Aggregate Purchase Price	$ 351.6
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares \| shares	926,192
Business Combination, Deferred Purchase Price	$ 100.0
Business Combination, Purchase Price Held, Percentage	10.00%

Represents the amount of aggregate purchase price in a business combination.

No definition available.

Name:	caty_BusinessCombinationAggregatePurchasePrice
Namespace Prefix:	caty_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of deferred cash outflow associated with the acquisition of business and will be released within one year of the merger.

+ References

No definition available.

+ Details

Name:	caty_BusinessCombinationDeferredPurchasePrice
Namespace Prefix:	caty_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The percentage of purchase price being held and will be released over three years following the closing of acquisition.

+ References

No definition available.

+ Details

Name:	caty_BusinessCombinationPurchasePriceHeldPercentage
Namespace Prefix:	caty_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares of equity interests issued or issuable to acquire entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)(4)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

+ Details

Name:	us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=caty_SinoPacBancorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: