Note 9 - Capital Resources (Details Textual) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	May 07, 2019	Dec. 31, 2017
Stockholders' Equity Attributable to Parent, Ending Balance	$ 2,418,144				$ 2,294,283				$ 2,418,144	$ 2,294,283	$ 2,121,866		$ 1,973,304
Stockholders' Equity, Period Increase (Decrease), Total									$ 123,900
Stockholders' Equity, Period Increase (Decrease), Percentage									5.40%
Net Income (Loss) Attributable to Parent, Total	$ 70,893	$ 56,794	$ 54,321	$ 46,852	$ 67,377	$ 72,835	$ 72,244	$ 66,679	$ 228,860	279,135	271,885
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan									9,777	3,366	2,822
Share-based Payment Arrangement, Expense									5,600
Other Comprehensive Income (Loss), Net of Tax, Total									3,008	20,308	(6,424)
Share-based Payment Arrangement, Decrease for Tax Withholding Obligation									1,911	2,311	2,440
Treasury Stock, Value, Acquired, Cost Method									23,593	36,301	42,648
Dividends, Common Stock, Cash									$ 98,688	$ 99,131	83,446
Common Stock, Dividends, Per Share, Cash Paid (in dollars per share)									$ 1.24	$ 1.24
May 2019 Repurchase Program [Member]
Treasury Stock, Value, Acquired, Cost Method									$ 50,000	$ 26,400
Stock Repurchase Program, Authorized Amount												$ 50,000
Treasury Stock, Shares, Acquired (in shares)									1,541,912	741,934
Treasury Stock Acquired, Average Cost Per Share (in dollars per share)									$ 32.43	$ 35.59
Restricted Stock Units (RSUs) [Member]
Share-based Payment Arrangement, Expense										$ 6,600	$ 7,300
Share-based Payment Arrangement, Decrease for Tax Withholding Obligation									$ 1,900