Note 9 - Capital Resources (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	May 31, 2019	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 07, 2019	Oct. 26, 2018	Dec. 31, 2016
Stockholders' Equity Attributable to Parent, Ending Balance		$ 2,294,283				$ 2,121,866				$ 2,294,283	$ 2,121,866	$ 1,973,304			$ 1,828,539
Stockholders' Equity, Period Increase (Decrease), Total										$ 172,400
Stockholders' Equity, Period Increase (Decrease), Percentage										8.10%
Net Income (Loss) Attributable to Parent, Total		67,377	$ 72,835	$ 72,244	$ 66,679	$ 64,648	$ 69,757	$ 73,664	$ 63,816	$ 279,135	271,885	176,042
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan										3,366	2,822	2,528
Share-based Payment Arrangement, Expense										6,600
Other Comprehensive Income (Loss), Net of Tax, Total										20,308	(6,424)	1,204
Share-based Payment Arrangement, Decrease for Tax Withholding Obligation										2,311	2,440	6,813
Treasury Stock, Value, Acquired, Cost Method										36,301	42,648
Dividends, Common Stock, Cash										$ 99,131	$ 83,446	69,888
Common Stock, Dividends, Per Share, Cash Paid										$ 1.24	$ 1.03
February 2016 Repurchase Program [Member]
Stock Repurchase Program, Authorized Amount														$ 45,000
October 2018 Repurchase Program [Member]
Treasury Stock, Value, Acquired, Cost Method	$ 45,000										$ 35,100
Treasury Stock, Shares, Acquired	1,182,060										913,400
Treasury Stock Acquired, Average Cost Per Share	$ 38.07										$ 38.43
May 2019 Repurchase Program [Member]
Treasury Stock, Value, Acquired, Cost Method										$ 26,400
Stock Repurchase Program, Authorized Amount													$ 50,000
Treasury Stock, Shares, Acquired										741,934
Treasury Stock Acquired, Average Cost Per Share										$ 35.59
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 23,600								$ 23,600
Restricted Stock Units (RSUs) [Member]
Share-based Payment Arrangement, Expense										6,600	$ 7,300	$ 5,200
Share-based Payment Arrangement, Decrease for Tax Withholding Obligation										$ 2,300