Note 11 - Capital Resources (Details) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended	3 Months Ended								12 Months Ended
	Feb. 16, 2016	Feb. 01, 2016	Jul. 31, 2015	Aug. 31, 2015	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2007	Nov. 30, 2007
Note 11 - Capital Resources (Details) [Line Items]
Stockholders' Equity Attributable to Parent					$ 1,747,778				$ 1,602,888				$ 1,747,778	$ 1,602,888
Stockholders' Equity, Period Increase (Decrease)													$ 144,900
Stockholders' Equity, Period Increase (Decrease), Percentage													9.00%
Net Income (Loss) Attributable to Parent					$ 41,446	$ 38,483	$ 45,200	$ 35,980	$ 35,578	$ 35,909	$ 35,084	$ 31,259	$ 161,109	$ 137,830	$ 123,143
Treasury Stock, Value, Acquired, Cost Method													59,400
Dividends, Common Stock, Cash													$ 45,300
Common Stock, Dividends, Per Share, Cash Paid (in Dollars per share)													$ 0.56	$ 0.29
Interest Expense, Subordinated Notes and Debentures													$ 3,000	$ 2,900	$ 3,000
November 2007 Repurchase Program [Member]
Note 11 - Capital Resources (Details) [Line Items]
Treasury Stock, Value, Acquired, Cost Method				$ 18,100
Stock Repurchase Program, Number of Shares Authorized to be Repurchased (in Shares)																	1,000,000
Treasury Stock, Shares, Acquired (in Shares)				622,500												377,500
Treasury Stock Acquired, Average Cost Per Share (in Dollars per share)				$ 29.08
August 2015 Repurchase Program [Member]
Note 11 - Capital Resources (Details) [Line Items]
Treasury Stock, Value, Acquired, Cost Method													$ 41,300
Stock Repurchase Program, Number of Shares Authorized to be Repurchased (in Shares)				2,000,000
Treasury Stock, Shares, Acquired (in Shares)													1,366,750
Treasury Stock Acquired, Average Cost Per Share (in Dollars per share)													$ 30.22
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased (in Shares)					633,250								633,250
Subsequent Event [Member] | August 2015 Repurchase Program [Member]
Note 11 - Capital Resources (Details) [Line Items]
Treasury Stock, Value, Acquired, Cost Method		$ 17,000
Treasury Stock, Shares, Acquired (in Shares)		633,250
Treasury Stock Acquired, Average Cost Per Share (in Dollars per share)		$ 26.82
Subsequent Event [Member] | February 2016 Repurchase Program [Member]
Note 11 - Capital Resources (Details) [Line Items]
Treasury Stock, Value, Acquired, Cost Method	$ 15,800
Treasury Stock, Shares, Acquired (in Shares)	579,543
Treasury Stock Acquired, Average Cost Per Share (in Dollars per share)	$ 27.22
Stock Repurchase Program, Authorized Amount		$ 45,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 29,200
Asia Bancshares [Member]
Note 11 - Capital Resources (Details) [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned			$ 82,900		$ 82,800								$ 82,800
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares (in Shares)			2,580,359
Payments to Acquire Businesses, Gross			$ 57,006										$ 57,006