Note 3 - Recent Accounting Pronouncements - Cumulative Effect of Changes (Details) - USD ($) $ in Thousands		Sep. 30, 2021	Jun. 30, 2021	[1]	Jan. 01, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Allowance for credit losses on loans		$ 131,945				$ 166,538
Deferred tax assets						85,610
Allowance for unfunded commitments						5,880
Stockholders' equity		$ 2,463,291	$ 2,452,357			$ 2,418,144	$ 2,374,723	$ 2,342,448	$ 2,294,283
Cumulative Effect, Period of Adoption, Adjustment [Member]
Allowance for credit losses on loans	[2]				$ (1,560)
Deferred tax assets	[2]				1,319
Allowance for unfunded commitments	[2]				6,018
Stockholders' equity	[2]				(3,140)
Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Allowance for credit losses on loans	[2]				164,978
Deferred tax assets	[2]				86,929
Allowance for unfunded commitments	[2]				11,898
Stockholders' equity	[2]				$ 2,415,004

[1] Balance sheet amounts previously reported for the impact of the initial adoption of CECL on January 1, 2021 have been corrected. The correction had an after-tax decrease to the cumulative effect of the change in accounting principle related to ASC326 of $2.3 million.

[2] Balance sheet amounts previously reported for the impact of the initial adoption of CECL have been corrected. The correction decreased the allowance for loan losses by $2.2 million, increased the deferred tax asset by $1.0 million, increased the allowance for unfunded credit commitments by $5.5 million and had an after-tax decrease to opening retained earnings of $2.3 million.