Note 3 - Investment Securities - Gross Unrealized Losses, Investment Securities Portfolio (Details) $ in Thousands	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Available for sale securities, less than 12 months, number of investment positions	4	82
Available for sale securities, less than 12 months, fair value	$ 11,746	$ 499,743
Available for sale securities, less than 12 months, unrealized losses	$ (67)	$ (4,111)
Available for sale securities, 12 months or longer, number of investment positions	121	219
Available for sale securities, 12 months or longer, fair value	$ 900,513	$ 1,549,153
Available for sale securities, 12 months or longer, unrealized losses	$ (10,662)	$ (56,269)
Available for sale securities, total, number of investment positions	125	301
Available for sale securities, total, fair value	$ 912,259	$ 2,048,896
Available for sale securities, total, unrealized losses	$ (10,729)	$ (60,380)
Held to Maturity securities, less than 12 months, number of investment positions	1	114
Held to Maturity securities, less than 12 months, fair value	$ 55	$ 92,154
Held to Maturity securities, less than 12 months, unrecognized losses	$ 0	$ (305)
Held to Maturity securities, 12 months or longer, number of investment positions	76	220
Held to Maturity securities, 12 months or longer, fair value	$ 360,934	$ 564,120
Held to Maturity securities, 12 months or longer, unrecognized losses	$ (3,880)	$ (16,551)
Held to Maturity securities, total, number of investment positions	77	334
Held to Maturity securities, total, fair value	$ 360,989	$ 656,274
Held to Maturity securities, total, unrecognized losses	$ (3,880)	$ (16,856)
US Government-sponsored Enterprises Debt Securities [Member]
Available for sale securities, less than 12 months, number of investment positions	0	1
Available for sale securities, less than 12 months, fair value	$ 0	$ 990
Available for sale securities, less than 12 months, unrealized losses	$ 0	$ (5)
Available for sale securities, 12 months or longer, number of investment positions	7	9
Available for sale securities, 12 months or longer, fair value	$ 77,714	$ 117,963
Available for sale securities, 12 months or longer, unrealized losses	$ (170)	$ (3,270)
Available for sale securities, total, number of investment positions	7	10
Available for sale securities, total, fair value	$ 77,714	$ 118,953
Available for sale securities, total, unrealized losses	$ (170)	$ (3,275)
Agency Residential MBS [Member]
Available for sale securities, less than 12 months, number of investment positions	0	8
Available for sale securities, less than 12 months, fair value	$ 0	$ 107,497
Available for sale securities, less than 12 months, unrealized losses	$ 0	$ (507)
Available for sale securities, 12 months or longer, number of investment positions	54	58
Available for sale securities, 12 months or longer, fair value	$ 533,982	$ 640,210
Available for sale securities, 12 months or longer, unrealized losses	$ (8,314)	$ (29,932)
Available for sale securities, total, number of investment positions	54	66
Available for sale securities, total, fair value	$ 533,982	$ 747,707
Available for sale securities, total, unrealized losses	$ (8,314)	$ (30,439)
Held to Maturity securities, less than 12 months, number of investment positions	1	16
Held to Maturity securities, less than 12 months, fair value	$ 55	$ 8,495
Held to Maturity securities, less than 12 months, unrecognized losses	$ 0	$ (34)
Held to Maturity securities, 12 months or longer, number of investment positions	63	78
Held to Maturity securities, 12 months or longer, fair value	$ 350,418	$ 412,574
Held to Maturity securities, 12 months or longer, unrecognized losses	$ (3,861)	$ (14,095)
Held to Maturity securities, total, number of investment positions	64	94
Held to Maturity securities, total, fair value	$ 350,473	$ 421,069
Held to Maturity securities, total, unrecognized losses	$ (3,861)	$ (14,129)
US Treasury Securities [Member]
Available for sale securities, less than 12 months, number of investment positions		2
Available for sale securities, less than 12 months, fair value		$ 54,805
Available for sale securities, less than 12 months, unrealized losses		$ (3)
Available for sale securities, 12 months or longer, number of investment positions		0
Available for sale securities, 12 months or longer, fair value		$ 0
Available for sale securities, 12 months or longer, unrealized losses		$ 0
Available for sale securities, total, number of investment positions		2
Available for sale securities, total, fair value		$ 54,805
Available for sale securities, total, unrealized losses		$ (3)
US States and Political Subdivisions Debt Securities [Member]
Available for sale securities, less than 12 months, number of investment positions	2	32
Available for sale securities, less than 12 months, fair value	$ 1,054	$ 26,452
Available for sale securities, less than 12 months, unrealized losses	$ 0	$ (166)
Available for sale securities, 12 months or longer, number of investment positions	25	71
Available for sale securities, 12 months or longer, fair value	$ 10,643	$ 67,121
Available for sale securities, 12 months or longer, unrealized losses	$ (100)	$ (2,819)
Available for sale securities, total, number of investment positions	27	103
Available for sale securities, total, fair value	$ 11,697	$ 93,573
Available for sale securities, total, unrealized losses	$ (100)	$ (2,985)
Held to Maturity securities, less than 12 months, number of investment positions	0	97
Held to Maturity securities, less than 12 months, fair value	$ 0	$ 83,633
Held to Maturity securities, less than 12 months, unrecognized losses	$ 0	$ (271)
Held to Maturity securities, 12 months or longer, number of investment positions	13	142
Held to Maturity securities, 12 months or longer, fair value	$ 10,516	$ 151,546
Held to Maturity securities, 12 months or longer, unrecognized losses	$ (19)	$ (2,456)
Held to Maturity securities, total, number of investment positions	13	239
Held to Maturity securities, total, fair value	$ 10,516	$ 235,179
Held to Maturity securities, total, unrecognized losses	$ (19)	$ (2,727)
Non-agency Residential MBS [Member]
Held to Maturity securities, less than 12 months, number of investment positions		1
Held to Maturity securities, less than 12 months, fair value		$ 26
Held to Maturity securities, less than 12 months, unrecognized losses		$ 0
Held to Maturity securities, 12 months or longer, number of investment positions		0
Held to Maturity securities, 12 months or longer, fair value		$ 0
Held to Maturity securities, 12 months or longer, unrecognized losses		$ 0
Held to Maturity securities, total, number of investment positions		1
Held to Maturity securities, total, fair value		$ 26
Held to Maturity securities, total, unrecognized losses		$ 0
Agency Commercial MBS [Member]
Available for sale securities, less than 12 months, number of investment positions	0	1
Available for sale securities, less than 12 months, fair value	$ 0	$ 1,842
Available for sale securities, less than 12 months, unrealized losses	$ 0	$ (27)
Available for sale securities, 12 months or longer, number of investment positions	1	0
Available for sale securities, 12 months or longer, fair value	$ 1,833	$ 0
Available for sale securities, 12 months or longer, unrealized losses	$ (19)	$ 0
Available for sale securities, total, number of investment positions	1	1
Available for sale securities, total, fair value	$ 1,833	$ 1,842
Available for sale securities, total, unrealized losses	$ (19)	$ (27)
US Government Agencies Debt Securities [Member]
Available for sale securities, less than 12 months, number of investment positions	0	0
Available for sale securities, less than 12 months, fair value	$ 0	$ 0
Available for sale securities, less than 12 months, unrealized losses	$ 0	$ 0
Available for sale securities, 12 months or longer, number of investment positions	2	2
Available for sale securities, 12 months or longer, fair value	$ 808	$ 1,119
Available for sale securities, 12 months or longer, unrealized losses	$ (7)	$ (9)
Available for sale securities, total, number of investment positions	2	2
Available for sale securities, total, fair value	$ 808	$ 1,119
Available for sale securities, total, unrealized losses	$ (7)	$ (9)
Debt Security, Corporate, US [Member]
Available for sale securities, less than 12 months, number of investment positions	2	38
Available for sale securities, less than 12 months, fair value	$ 10,692	$ 308,157
Available for sale securities, less than 12 months, unrealized losses	$ (67)	$ (3,403)
Available for sale securities, 12 months or longer, number of investment positions	32	79
Available for sale securities, 12 months or longer, fair value	$ 275,533	$ 722,740
Available for sale securities, 12 months or longer, unrealized losses	$ (2,052)	$ (20,239)
Available for sale securities, total, number of investment positions	34	117
Available for sale securities, total, fair value	$ 286,225	$ 1,030,897
Available for sale securities, total, unrealized losses	$ (2,119)	$ (23,642)