Note 19 - Condensed Financial Information of Cathay General Bancorp - Balance Sheets (Details) (Parentheticals) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
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)	89,104,022	87,820,920
Common stock, shares outstanding (in shares)	80,893,379	79,610,277
Treasury stock shares (in shares)	8,210,643	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)	89,104,022	87,820,920
Common stock, shares outstanding (in shares)	80,893,379	79,610,277
Treasury stock shares (in shares)	8,210,643	8,210,643