Note 20 - Condensed Financial Information of Cathay General Bancorp - Balance Sheets (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 31, 2014
Assets
Cash pledged as margin for interest rate swaps	$ 1,800,000
Short-term certificates of deposit	374,957,000	$ 292,745,000
Securities available for sale	1,242,509,000	1,333,626,000
Other assets	175,419,000	167,491,000
Total assets	16,784,737,000	15,640,186,000
Liabilities
Junior subordinated debt	119,136,000				$ 119,100,000
Long-term debt	189,448,000	194,136,000
Deferred payments from acquisition	18,458,000	35,404,000
Other liabilities	182,618,000	186,486,000
Total liabilities	14,662,871,000	13,666,882,000
Commitments and contingencies
Stockholders' equity
Common stock, $0.01 par value, 100,000,000 shares authorized, 89,826,317 issued and 80,501,948 outstanding at December 31, 2018, and 89,104,022 issued and 80,893,379 outstanding at December 31, 2017	898,000	891,000
Additional paid-in-capital	942,062,000	932,874,000
Accumulated other comprehensive loss, net	(18,006,000)	(2,511,000)
Retained earnings	1,479,149,000	1,281,639,000
Treasury stock, at cost (9,324,369 shares at December 31, 2018, and 8,210,643 shares at December 31, 2017)	282,237,000	239,589,000
Total stockholders' equity	2,121,866,000	1,973,304,000	$ 1,828,539,000	$ 1,747,778,000
Total liabilities and stockholders' equity	16,784,737,000	15,640,186,000
Parent Company [Member]
Assets
Cash	19,861,000	44,645,000
Cash pledged as margin for interest rate swaps	1,766,000	4,506,000
Short-term certificates of deposit	328,000	327,000
Securities available for sale	17,564,000	19,806,000
Other assets	6,012,000	6,831,000
Total assets	2,340,256,000	2,215,359,000
Liabilities
Junior subordinated debt	119,136,000	119,136,000
Long-term debt	70,313,000	75,000,000
Deferred payments from acquisition	18,458,000	35,404,000
Other liabilities	10,483,000	12,515,000
Total liabilities	218,390,000	242,055,000
Commitments and contingencies
Stockholders' equity
Common stock, $0.01 par value, 100,000,000 shares authorized, 89,826,317 issued and 80,501,948 outstanding at December 31, 2018, and 89,104,022 issued and 80,893,379 outstanding at December 31, 2017	898,000	891,000
Additional paid-in-capital	942,062,000	932,874,000
Accumulated other comprehensive loss, net	(18,006,000)	(2,511,000)
Retained earnings	1,479,149,000	1,281,639,000
Treasury stock, at cost (9,324,369 shares at December 31, 2018, and 8,210,643 shares at December 31, 2017)	(282,237,000)	(239,589,000)
Total stockholders' equity	2,121,866,000	1,973,304,000
Total liabilities and stockholders' equity	2,340,256,000	2,215,359,000
Parent Company [Member] | Bank Subsidiaries [Member]
Assets
Investment in Cathay Bank subsidiary	2,290,073,000	2,134,445,000
Investment in subsidiaries	2,290,073,000	2,134,445,000
Parent Company [Member] | Non Bank Subsidiaries [Member]
Assets
Investment in Cathay Bank subsidiary	4,652,000	4,799,000
Investment in subsidiaries	$ 4,652,000	$ 4,799,000