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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2015
Interest income	$ 155,640	$ 154,078	$ 135,629	$ 130,804	$ 130,668	$ 124,155	$ 121,902	$ 122,345
Interest expense	22,593	20,882	18,277	18,690	20,766	20,331	20,126	19,977	$ 80,442	$ 81,200	$ 73,964
Net interest income	133,047	133,196	117,352	112,114	109,902	103,824	101,776	102,368	495,709	417,870	379,742
Provision/(reversal) for loan losses				(2,500)			(5,150)	(10,500)	(2,500)	(15,650)	(11,400)
Net-interest income after reversal for loan losses	133,047	133,196	117,352	114,614	109,902	103,824	106,926	112,868	498,209	433,520	391,142
Non-interest income	10,466	12,961	6,152	6,718	7,961	8,811	9,057	7,541	36,297	33,370	32,674
Non-interest expense	66,407	61,248	56,658	51,886	53,503	50,737	68,879	51,571	236,199	224,690	202,720
Income before income tax expense	77,106	84,909	66,846	69,446	64,360	61,898	47,104	68,838	298,307	242,200	221,096
Income tax benefit	51,166	35,163	15,431	20,505	16,345	15,808	12,273	22,675	122,265	67,101	59,987	$ 59,987
Net income	$ 25,940	$ 49,746	$ 51,415	$ 48,941	$ 48,015	$ 46,090	$ 34,831	$ 46,163	$ 176,042	$ 175,099	$ 161,109
Basic (in dollars per share)	$ 0.32	$ 0.62	$ 0.64	$ 0.61	$ 0.61	$ 0.58	$ 0.44	$ 0.58	$ 2.19	$ 2.21	$ 2
Diluted (in dollars per share)	$ 0.32	$ 0.61	$ 0.64	$ 0.61	$ 0.60	$ 0.58	$ 0.44	$ 0.57	$ 2.17	$ 2.19	$ 1.98