Note 19 - Condensed Financial Information of Cathay General Bancorp - Balance Sheets (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	May 31, 2014
Assets
Cash pledged as margin for interest rate swaps	$ 4,500,000
Short-term certificates of deposit	292,745,000	$ 967,067,000
Securities available-for-sale (amortized cost of $1,336,345 in 2017 and $1,317,012 in 2016)	1,333,626,000	1,314,345,000
Other assets	167,491,000	117,902,000
Total assets	15,640,186,000	14,520,769,000
Liabilities
Junior subordinated debt	119,136,000				$ 119,100,000
Long-term debt	194,136,000	119,136,000
Deferred payments from acquisition	35,404,000
Other liabilities	186,486,000	168,524,000
Total liabilities	13,666,882,000	12,692,230,000
Commitments and contingencies
Stockholders' equity
Common stock, $0.01 par value, 100,000,000 shares authorized, 89,104,022 issued and 80,893,379 outstanding at December 31, 2017, and 87,820,920 issued and 79,610,277 outstanding at December 31, 2016	891,000	878,000
Additional paid-in-capital	932,874,000	895,480,000
Accumulated other comprehensive loss, net	(2,511,000)	(3,715,000)
Retained earnings	1,281,639,000	1,175,485,000
Treasury stock, at cost (8,210,643 shares at December 31, 2017, and at December 31, 2016)	239,589,000	239,589,000
Total stockholders' equity	1,973,304,000	1,828,539,000	$ 1,747,778,000	$ 1,602,888,000
Total liabilities and stockholders' equity	15,640,186,000	14,520,769,000
Parent Company [Member]
Assets
Cash	44,645,000	34,596,000
Cash pledged as margin for interest rate swaps	4,506,000	6,895,000
Short-term certificates of deposit	327,000	325,000
Securities available-for-sale (amortized cost of $1,336,345 in 2017 and $1,317,012 in 2016)	19,806,000	18,129,000
Other assets	6,831,000	6,674,000
Total assets	2,215,359,000	1,951,935,000
Liabilities
Junior subordinated debt	119,136,000	119,136,000
Long-term debt	75,000,000
Deferred payments from acquisition	35,404,000
Other liabilities	12,515,000	4,260,000
Total liabilities	242,055,000	123,396,000
Commitments and contingencies
Stockholders' equity
Common stock, $0.01 par value, 100,000,000 shares authorized, 89,104,022 issued and 80,893,379 outstanding at December 31, 2017, and 87,820,920 issued and 79,610,277 outstanding at December 31, 2016	891,000	878,000
Additional paid-in-capital	932,874,000	895,480,000
Accumulated other comprehensive loss, net	(2,511,000)	(3,715,000)
Retained earnings	1,281,639,000	1,175,485,000
Treasury stock, at cost (8,210,643 shares at December 31, 2017, and at December 31, 2016)	(239,589,000)	(239,589,000)
Total stockholders' equity	1,973,304,000	1,828,539,000
Total liabilities and stockholders' equity	2,215,359,000	1,951,935,000
Parent Company [Member] | Bank Subsidiaries [Member]
Assets
Investment in Cathay Bank subsidiary	2,134,445,000	1,879,868,000
Investment in subsidiaries	2,134,445,000	1,879,868,000
Parent Company [Member] | Non Bank Subsidiaries [Member]
Assets
Investment in Cathay Bank subsidiary	4,799,000	5,448,000
Investment in subsidiaries	$ 4,799,000	$ 5,448,000