Note 4 - Earnings per Share (Detail) - Earnings per Share (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Net income attributable to Cathay General Bancorp (in Dollars)	$ 28,847	$ 28,913
Dividends on preferred stock and noncash charge from repayment (in Dollars)	(5,184)	(4,117)
Net income available to common stockholders (in Dollars)	$ 23,663	$ 24,796
Basic weighted-average number of common shares outstanding	78,795,564	78,678,645
Dilutive effect of weighted-average outstanding common share equivalents
Stock options	19,577	11,487
Diluted weighted-average number of common shares outstanding	78,815,141	78,690,132
Average stock options and warrants with anti-dilutive effect	5,630,813	6,227,224
Basic (in Dollars per share)	$ 0.30	$ 0.32
Diluted (in Dollars per share)	$ 0.30	$ 0.32