Note 3 - Investment Securities (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Proceeds from Sale of Debt Securities, Available-for-sale	$ 117,200	$ 293,800
Proceeds from Maturities, Prepayments and Calls of Debt Securities, Available-for-sale	734,485	296,721	$ 447,773
Debt Securities, Available-for-sale, Realized Gain	1,700	583
Debt Securities, Available-for-sale, Realized Loss	$ 0	372
Temporarily Impaired Securities As Percentage Of Investment Securities	17.90%
Percentage Of Unrealized Losses For Securities With Unrealized Losses For Less Than Twelve Months	0.30%
Percentage Of Unrealized Losses For Securities With Unrealized Losses For Greater Than Twelve Months	2.10%
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 1,459	1,740
Pledged Investment Securities	22,700	20,100
Stockholders' Equity Attributable to Parent, Ending Balance	2,418,144	2,294,283	2,121,866		$ 1,973,304
Equity Securities, FV-NI, Unrealized Gain (Loss), Total	(1,148)	5,736	(2,787)
Equity Securities, FV-NI	23,744	28,005
Retained Earnings [Member]
Stockholders' Equity Attributable to Parent, Ending Balance	$ 1,789,325	$ 1,659,153	$ 1,479,149		$ 1,281,639
Accounting Standards Update 2016-01 [Member] | Retained Earnings [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Stockholders' Equity Attributable to Parent, Ending Balance				$ 8,600