Note 10 - Capital Resources (Details Textual) - USD ($)	1 Months Ended	2 Months Ended	3 Months Ended								12 Months Ended
	Oct. 31, 2018	Feb. 29, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Oct. 26, 2018	Feb. 01, 2016	Dec. 31, 2015
Stockholders' Equity Attributable to Parent, Ending Balance			$ 2,121,866,000				$ 1,973,304,000				$ 2,121,866,000	$ 1,973,304,000	$ 1,828,539,000			$ 1,747,778,000
Stockholders' Equity, Period Increase (Decrease), Total											$ 148,600,000
Stockholders' Equity, Period Increase (Decrease), Percentage											7.50%
Net Income (Loss) Attributable to Parent, Total			$ 64,648,000	$ 69,757,000	$ 73,664,000	$ 63,816,000	$ 25,940,000	$ 49,746,000	$ 51,415,000	$ 48,941,000	$ 271,885,000	176,042,000	175,099,000
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan											2,822,000	2,528,000	2,277,000
Proceeds from Stock Options Exercised											838,000	1,094,000	7,661,000
Treasury Stock, Value, Acquired, Cost Method											42,648,000		54,441,000
Adjustments Related to Tax Withholding for Share-based Compensation											2,440,000	6,813,000	103,000
Dividends, Common Stock, Cash											$ 83,446,000	$ 69,888,000	59,274,000
Common Stock, Dividends, Per Share, Cash Paid											$ 1.03	$ 0.87
August 2015 Repurchase Program [Member]
Treasury Stock, Value, Acquired, Cost Method		$ 17,000,000
Treasury Stock, Shares, Acquired		633,250
Stock Repurchase Program, Number of Shares Authorized to be Repurchased		2,000,000
Treasury Stock Acquired, Average Cost Per Share		$ 26.82
February 2016 Repurchase Program [Member]
Treasury Stock, Value, Acquired, Cost Method	$ 7,500,000												$ 37,500,000
Treasury Stock, Shares, Acquired	200,326												1,380,578
Treasury Stock Acquired, Average Cost Per Share	$ 37.65												$ 27.13
Stock Repurchase Program, Authorized Amount															$ 45,000,000
October 2018 Repurchase Program [Member]
Treasury Stock, Value, Acquired, Cost Method											$ 35,100,000
Treasury Stock, Shares, Acquired											913,400
Treasury Stock Acquired, Average Cost Per Share											$ 38.43
Stock Repurchase Program, Authorized Amount														$ 45,000,000
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased			9,900,000								9,900,000
Restricted Stock Units (RSUs) [Member]
Adjustments Related to Tax Withholding for Share-based Compensation											$ 2,400,000