Note 9 - Capital Resources (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	4 Months Ended		12 Months Ended
	Dec. 31, 2021	Aug. 05, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Sep. 02, 2021	Apr. 01, 2021	Dec. 31, 2018
Stockholders' Equity Attributable to Parent, Ending Balance	$ 2,446,251		$ 2,446,251	$ 2,418,144	$ 2,294,283			$ 2,121,866
Stockholders' Equity, Period Increase (Decrease), Total			$ 28,100
Stockholders' Equity, Period Increase (Decrease), Percentage			1.20%
Net Income (Loss) Attributable to Parent, Total			$ 298,304	228,860	279,135
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan			3,563	9,777	3,366
Share-based Payment Arrangement, Expense			6,000
Other Comprehensive Income (Loss), Net of Tax, Total			(8,375)	3,008	20,308
Share-based Payment Arrangement, Decrease for Tax Withholding Obligation			2,632	1,911	2,311
Treasury Stock, Value, Acquired, Cost Method	$ 92,100	$ 75,000	167,104	23,593	36,301
Dividends, Common Stock, Cash			$ 99,322	$ 98,688	$ 99,131
Common Stock, Dividends, Per Share, Cash Paid (in dollars per share)			$ 1.27	$ 1.24	$ 1.24
Stock Repurchase Program, Authorized Amount						$ 125,000	$ 75,000
Treasury Stock, Shares, Acquired (in shares)	2,153,576	1,832,481	3,986,057
Treasury Stock Acquired, Average Cost Per Share (in dollars per share)	$ 42.77	$ 40.93	$ 41.92
Restricted Stock Units (RSUs) [Member]
Share-based Payment Arrangement, Expense			$ 6,000	$ 5,600	$ 6,600
Share-based Payment Arrangement, Decrease for Tax Withholding Obligation			$ 2,600