Note 19 - Condensed Financial Information of Cathay General Bancorp - Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 31, 2014
Assets
Cash pledged as margin for interest rate swaps	$ 34,700	$ 17,700
Equity securities	23,744	28,005
Other assets	170,889	150,924
Total assets	19,043,134	18,094,144
Liabilities
Junior subordinated debt	119,136				$ 119,100
Deferred payments from acquisition	0	7,644
Other liabilities	175,502	209,501
Total liabilities	16,624,990	15,799,861
Commitments and contingencies	0	0
Stockholders' equity
Common stock, $0.01 par value, 100,000,000 shares authorized, 90,643,206 issued and 79,508,265 outstanding at December 31, 2020, and 90,064,382 issued and 79,729,419 outstanding at December 31, 2019	906	900
Additional paid-in-capital	964,734	950,466
Accumulated other comprehensive income, net	5,310	2,302
Retained earnings	1,789,325	1,659,153
Treasury stock, at cost (11,134,941 shares at December 31, 2020, and 10,334,963 shares at December 31, 2019)	(342,131)	(318,538)
Total stockholders' equity	2,418,144	2,294,283	$ 2,121,866	$ 1,973,304
Total liabilities and stockholders' equity	19,043,134	18,094,144
Parent Company [Member]
Assets
Cash	50,060	32,468
Cash pledged as margin for interest rate swaps	2,159	7,098
Short-term certificates of deposit	332	330
Equity securities	15,505	19,258
Other assets	6,447	5,530
Total assets	2,542,991	2,434,348
Liabilities
Junior subordinated debt	119,136	119,136
Deferred payments from acquisition	0	7,644
Other liabilities	5,711	13,285
Total liabilities	124,847	140,065
Commitments and contingencies	0	0
Stockholders' equity
Common stock, $0.01 par value, 100,000,000 shares authorized, 90,643,206 issued and 79,508,265 outstanding at December 31, 2020, and 90,064,382 issued and 79,729,419 outstanding at December 31, 2019	906	900
Additional paid-in-capital	964,734	950,466
Accumulated other comprehensive income, net	5,310	2,302
Retained earnings	1,789,325	1,659,153
Treasury stock, at cost (11,134,941 shares at December 31, 2020, and 10,334,963 shares at December 31, 2019)	(342,131)	(318,538)
Total stockholders' equity	2,418,144	2,294,283
Total liabilities and stockholders' equity	2,542,991	2,434,348
Parent Company [Member] | Bank Subsidiaries [Member]
Assets
Investment in Cathay Bank subsidiary	2,467,643	2,365,206
Investment in subsidiaries	2,467,643	2,365,206
Parent Company [Member] | Non Bank Subsidiaries [Member]
Assets
Investment in Cathay Bank subsidiary	845	4,458
Investment in subsidiaries	$ 845	$ 4,458