Note 23 - Quarterly Results of Operations (Unaudited) - Selected Unaudited Quarterly Financial Data (Details) - 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
Interest income	$ 167,596	$ 172,234	$ 174,008	$ 186,736	$ 191,226	$ 197,831	$ 192,449	$ 187,761
Interest expense	27,776	34,730	39,533	46,425	50,015	50,831	49,070	44,445	$ 148,464	$ 194,361	$ 122,056
Net interest income	139,820	137,504	134,475	140,311	141,211	147,000	143,379	143,316	552,110	574,906	565,899
(Reversal)/provision for credit losses	(5,000)	12,500	25,000	25,000	(5,000)	(2,000)	0	0	57,500	(7,000)	(4,500)
Net-interest income after reversal for loan losses	144,820	125,004	109,475	115,311	146,211	149,000	143,379	143,316	494,610	581,906	570,399
Non-interest income	11,451	9,977	15,606	5,786	8,648	10,388	12,794	12,921	42,820	44,751	31,707
Non-interest expense	75,046	75,997	67,268	65,154	71,192	65,580	69,546	70,970	283,465	277,288	264,419
Income before income tax expense	81,225	58,984	57,813	55,943	83,667	93,808	86,627	85,267
Income tax benefit	10,332	2,190	3,492	9,091	16,290	20,973	14,383	18,588	25,105	70,234	65,802
Net income	$ 70,893	$ 56,794	$ 54,321	$ 46,852	$ 67,377	$ 72,835	$ 72,244	$ 66,679	$ 228,860	$ 279,135	$ 271,885
Basic (in dollars per share)	$ 0.89	$ 0.71	$ 0.68	$ 0.59	$ 0.85	$ 0.91	$ 0.90	$ 0.83	$ 2.88	$ 3.49	$ 3.35
Diluted (in dollars per share)	$ 0.89	$ 0.71	$ 0.68	$ 0.59	$ 0.84	$ 0.91	$ 0.90	$ 0.83	$ 2.87	$ 3.48	$ 3.33