Note 4 - Loans - Activity in the Allowance for Credit Losses (Details) - USD ($) $ 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
Balance at beginning of year				$ 123,224				$ 122,391	$ 123,224	$ 122,391	$ 123,279
Provision/(reversal) for credit losses	$ (5,000)	$ 12,500	$ 25,000	25,000	$ (5,000)	$ (2,000)	$ 0	0	57,500	(7,000)	(4,500)
Loans charged off									(21,996)	(6,997)	(3,206)
Recoveries of charged off loans									7,810	14,830	6,818
Balance at end of year	166,538				123,224				166,538	123,224	122,391
Reserve for off Balance Sheet Credit Commitments [Member]
Balance at beginning of year				$ 3,855				$ 2,250	3,855	2,250	4,588
Provision/(reversal) for credit losses									2,025	1,605	(2,338)
Balance at end of year	$ 5,880				$ 3,855				$ 5,880	$ 3,855	$ 2,250