Note 20 - Condensed Financial Information of Cathay General Bancorp (Detail) - Balance Sheets (Parentheticals) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Preferred stock, shares authorized (in Shares)	10,000,000	10,000,000
Preferred stock, shares issued (in Shares)	258,000	258,000
Preferred stock, shares outstanding (in Shares)	258,000	258,000
Common stock, par value (in Dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in Shares)	100,000,000	100,000,000
Common stock, shares issued (in Shares)	82,985,853	82,860,122
Common stock, shares outstanding (in Shares)	78,778,288	78,652,557
Treasury stock, shares (in Shares)	4,207,565	4,207,565
Parent Company [Member]
Preferred stock, shares authorized (in Shares)	10,000,000	10,000,000
Preferred stock, shares issued (in Shares)	258,000	258,000
Preferred stock, shares outstanding (in Shares)	258,000	258,000
Common stock, par value (in Dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in Shares)	100,000,000	100,000,000
Common stock, shares issued (in Shares)	82,985,853	82,860,122
Common stock, shares outstanding (in Shares)	78,778,288	78,652,557
Treasury stock, shares (in Shares)	4,207,565	4,207,565