Note 10 - Capital Resources (Details Textual) - USD ($)	1 Months Ended	2 Months Ended	3 Months Ended								12 Months Ended
	Aug. 31, 2015	Feb. 29, 2016	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Feb. 01, 2016	Dec. 31, 2013
Stockholders' Equity Attributable to Parent			$ 1,828,539,000				$ 1,747,778,000				$ 1,828,539,000	$ 1,747,778,000	$ 1,602,888,000		$ 1,458,971,000
Stockholders' Equity, Period Increase (Decrease)											$ 80,800,000
Stockholders' Equity, Period Increase (Decrease), Percentage											4.60%
Net Income (Loss) Attributable to Parent			$ 48,015,000	$ 46,090,000	$ 34,831,000	$ 46,163,000	$ 41,446,000	$ 38,483,000	$ 45,200,000	$ 35,980,000	$ 175,099,000	161,109,000	137,830,000
Other Comprehensive Income (Loss), Net of Tax											4,711,000	(2,857,000)	24,160,000
Proceeds from Stock Options Exercised											7,661,000	5,014,000	128,000
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan											2,277,000	4,175,000	2,848,000
Treasury Stock, Value, Acquired, Cost Method											54,441,000	59,412,000
Dividends, Common Stock, Cash											$ 59,274,000	$ 45,283,000	23,105,000
Common Stock, Dividends, Per Share, Cash Paid											$ 0.75	$ 0.56
Interest Expense, Subordinated Notes and Debentures											$ 3,500,000	$ 3,000,000	$ 2,900,000
U.S. Treasury [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights			943,345				1,846,374				943,345	1,846,374
Class of Warrant or Right, Exercise Price of Warrants or Rights			$ 20.65				$ 20.96				$ 20.65	$ 20.96
Class of Warrant or Right, Increase in Warrants Exercisable, Percentage											1.50%
Number of Warrants Exercised											930,113
Stock Issued During the Period, Warrants Exercised											388,001
November 2007 [Member]
Treasury Stock, Value, Acquired, Cost Method	$ 18,100,000
Treasury Stock, Shares, Acquired	622,500
Treasury Stock Acquired, Average Cost Per Share	$ 29.08
August 2015 Repurchase Program [Member]
Treasury Stock, Value, Acquired, Cost Method		$ 17,000,000
Treasury Stock, Shares, Acquired		633,250										1,366,750
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, Value, Acquired, Cost Method											$ 37,500,000
Treasury Stock, Shares, Acquired											1,380,578
Treasury Stock Acquired, Average Cost Per Share											$ 27.13
Stock Repurchase Program, Number of Shares Authorized to be Repurchased														45,000,000
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased			7,500,000								7,500,000