Note 20 - Condensed Financial Information of Cathay General Bancorp - Balance Sheets (Details) (Parentheticals) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
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)	90,064,382	89,826,317
Common stock, shares outstanding (in shares)	79,729,419	80,501,948
Treasury stock shares (in shares)	10,334,963	9,324,369
Parent Company [Member]
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)	90,064,382	89,826,317
Common stock, shares outstanding (in shares)	79,729,419	80,501,948
Treasury stock shares (in shares)	10,334,963	9,324,369