Note 10 - Capital Resources (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	2 Months Ended	3 Months Ended								12 Months Ended
	Aug. 31, 2015	Feb. 29, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 01, 2016	Dec. 31, 2014
Stockholders' Equity Attributable to Parent			$ 1,973,304				$ 1,828,539				$ 1,973,304	$ 1,828,539	$ 1,747,778		$ 1,602,888
Stockholders' Equity, Period Increase (Decrease)											$ 144,800
Stockholders' Equity, Period Increase (Decrease), Percentage											7.90%
Net Income (Loss) Attributable to Parent			$ 25,940	$ 49,746	$ 51,415	$ 48,941	$ 48,015	$ 46,090	$ 34,831	$ 46,163	$ 176,042	175,099	161,109
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan											2,528	2,277	4,175
Proceeds from Stock Options Exercised											1,094	7,661	5,014
Other Comprehensive Income (Loss), Net of Tax											1,204	4,711	(2,857)
Adjustments Related to Tax Withholding for Share-based Compensation											6,813	103	227
Dividends, Common Stock, Cash											$ 69,888	$ 59,274	45,283
Common Stock, Dividends, Per Share, Cash Paid											$ 0.87	$ 0.75
Treasury Stock, Value, Acquired, Cost Method												$ 54,441	59,412
Interest Expense, Subordinated Notes and Debentures											$ 4,100	$ 3,500	$ 3,000
November 2007 [Member]
Treasury Stock, Shares, Acquired	622,500
Treasury Stock, Value, Acquired, Cost Method	$ 18,100
Treasury Stock Acquired, Average Cost Per Share	$ 29.08
August 2015 Repurchase Program [Member]
Treasury Stock, Shares, Acquired		633,250											1,366,750
Treasury Stock, Value, Acquired, Cost Method		$ 17,000
Treasury Stock Acquired, Average Cost Per Share		$ 26.82
Stock Repurchase Program, Number of Shares Authorized to be Repurchased	2,000,000
February 2016 Repurchase Program [Member]
Treasury Stock, Shares, Acquired												1,380,578
Treasury Stock, Value, Acquired, Cost Method												$ 37,500
Treasury Stock Acquired, Average Cost Per Share												$ 27.13
Stock Repurchase Program, Authorized Amount														$ 45,000
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased			7,500,000								7,500,000
U.S. Treasury [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights			943,327				943,345				943,327	943,345	1,846,374
Class of Warrant or Right, Exercise Price of Warrants or Rights			$ 20.41								$ 20.41		$ 20.96
Class of Warrant or Right, Increase in Warrants Exercisable, Percentage											1.03%
Restricted Stock Units (RSUs) [Member]
Adjustments Related to Tax Withholding for Share-based Compensation											$ 6,800
SinoPac Bancorp [Member]
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned			$ 34,900								$ 34,900