Note 3 - Investment Securities - Securities in a Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available for sale securities in a continuous unrealized loss position, less than 12 months, fair value	$ 185,784	$ 269,895
Available for sale securities in a continuous unrealized loss position, less than 12 months, unrealized losses	1,818	8,777
Available for sale securities in a continuous unrealized loss position, 12 months or more, fair value	246,865	38,473
Available for sale securities in a continuous unrealized loss position, 12 months or more, unrealized losses	9,502	2,216
Available for sale securities in a continuous unrealized loss position, fair value	432,649	308,368
Available for sale securities in a continuous unrealized loss position, unrealized losses	11,320	10,993
Held to maturity securities in a continuous unrealized loss position, less than 12 months, fair value	154,170	274,424
Held to maturity securities in a continuous unrealized loss position, less than 12 months, unrealized losses	1,201	7,994
Held to maturity securities in a continuous unrealized loss position, 12 months or more, fair value	250,059	12,158
Held to maturity securities in a continuous unrealized loss position, 12 months or more, unrealized losses	7,624	124
Held to maturity securities in a continuous unrealized loss position, fair value	404,229	286,582
Held to maturity securities in a continuous unrealized loss position, unrealized losses	8,825	8,118
Collateralized Mortgage Obligations [Member]
Available for sale securities in a continuous unrealized loss position, less than 12 months, fair value	150,075	192,308
Available for sale securities in a continuous unrealized loss position, less than 12 months, unrealized losses	1,565	5,380
Available for sale securities in a continuous unrealized loss position, 12 months or more, fair value	170,166	7,579
Available for sale securities in a continuous unrealized loss position, 12 months or more, unrealized losses	6,166	252
Available for sale securities in a continuous unrealized loss position, fair value	320,241	199,887
Available for sale securities in a continuous unrealized loss position, unrealized losses	7,731	5,632
Held to maturity securities in a continuous unrealized loss position, less than 12 months, fair value	56,955	108,974
Held to maturity securities in a continuous unrealized loss position, less than 12 months, unrealized losses	767	2,469
Held to maturity securities in a continuous unrealized loss position, 12 months or more, fair value	107,986	8,572
Held to maturity securities in a continuous unrealized loss position, 12 months or more, unrealized losses	3,203	84
Held to maturity securities in a continuous unrealized loss position, fair value	164,941	117,546
Held to maturity securities in a continuous unrealized loss position, unrealized losses	3,970	2,553
US Government Agencies Debt Securities [Member]
Held to maturity securities in a continuous unrealized loss position, less than 12 months, fair value	42,045	67,725
Held to maturity securities in a continuous unrealized loss position, less than 12 months, unrealized losses	213	2,198
Held to maturity securities in a continuous unrealized loss position, 12 months or more, fair value	59,594	3,586
Held to maturity securities in a continuous unrealized loss position, 12 months or more, unrealized losses	2,022	40
Held to maturity securities in a continuous unrealized loss position, fair value	101,639	71,311
Held to maturity securities in a continuous unrealized loss position, unrealized losses	2,235	2,238
Agency Mortgage-backed Securities [Member]
Available for sale securities in a continuous unrealized loss position, less than 12 months, fair value	29,967	29,916
Available for sale securities in a continuous unrealized loss position, less than 12 months, unrealized losses	226	1,260
Available for sale securities in a continuous unrealized loss position, 12 months or more, fair value	21,045	3,199
Available for sale securities in a continuous unrealized loss position, 12 months or more, unrealized losses	574	35
Available for sale securities in a continuous unrealized loss position, fair value	51,012	33,115
Available for sale securities in a continuous unrealized loss position, unrealized losses	800	1,295
Held to maturity securities in a continuous unrealized loss position, less than 12 months, fair value	55,170	97,725
Held to maturity securities in a continuous unrealized loss position, less than 12 months, unrealized losses	221	3,327
Held to maturity securities in a continuous unrealized loss position, 12 months or more, fair value	82,479
Held to maturity securities in a continuous unrealized loss position, 12 months or more, unrealized losses	2,399
Held to maturity securities in a continuous unrealized loss position, fair value	137,649	97,725
Held to maturity securities in a continuous unrealized loss position, unrealized losses	2,620	3,327
US States and Political Subdivisions Debt Securities [Member]
Available for sale securities in a continuous unrealized loss position, less than 12 months, fair value	5,742	15,414
Available for sale securities in a continuous unrealized loss position, less than 12 months, unrealized losses	27	429
Available for sale securities in a continuous unrealized loss position, 12 months or more, fair value	2,656
Available for sale securities in a continuous unrealized loss position, 12 months or more, unrealized losses	35
Available for sale securities in a continuous unrealized loss position, fair value	8,398	15,414
Available for sale securities in a continuous unrealized loss position, unrealized losses	62	429
Corporate Debt Securities [Member]
Available for sale securities in a continuous unrealized loss position, less than 12 months, fair value		32,257
Available for sale securities in a continuous unrealized loss position, less than 12 months, unrealized losses		1,708
Available for sale securities in a continuous unrealized loss position, 12 months or more, fair value	52,509	10,726
Available for sale securities in a continuous unrealized loss position, 12 months or more, unrealized losses	2,491	270
Available for sale securities in a continuous unrealized loss position, fair value	52,509	42,983
Available for sale securities in a continuous unrealized loss position, unrealized losses	2,491	1,978
Asset-backed Securities [Member]
Available for sale securities in a continuous unrealized loss position, less than 12 months, fair value
Available for sale securities in a continuous unrealized loss position, less than 12 months, unrealized losses
Available for sale securities in a continuous unrealized loss position, 12 months or more, fair value		15,149
Available for sale securities in a continuous unrealized loss position, 12 months or more, unrealized losses		416
Available for sale securities in a continuous unrealized loss position, fair value		15,149
Available for sale securities in a continuous unrealized loss position, unrealized losses		416
Collateralized Debt Obligations [Member]
Available for sale securities in a continuous unrealized loss position, less than 12 months, fair value
Available for sale securities in a continuous unrealized loss position, less than 12 months, unrealized losses
Available for sale securities in a continuous unrealized loss position, 12 months or more, fair value	489	1,820
Available for sale securities in a continuous unrealized loss position, 12 months or more, unrealized losses	236	1,243
Available for sale securities in a continuous unrealized loss position, fair value	489	1,820
Available for sale securities in a continuous unrealized loss position, unrealized losses	$ 236	$ 1,243