Note 20 - Condensed Financial Information of Cathay General Bancorp - Balance Sheets (Details) (Parentheticals) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
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)	89,826,317	89,104,022
Common stock, shares outstanding (in shares)	80,501,948	80,893,379
Treasury stock shares (in shares)	9,324,369	8,210,643
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)	89,826,317	89,104,022
Common stock, shares outstanding (in shares)	80,501,948	80,893,379
Treasury stock shares (in shares)	9,324,369	8,210,643