Note 3 - Investment Securities - Securities With Unrealized Losses (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($)
Held-to-maturity, less than 12 months, estimated fair value	$ 28,341
Held-to-maturity, less than 12 months, gross unrealized losses	(136)
Held-to-maturity, 12 months or more, estimated fair value	49,234
Held-to-maturity, 12 months or more, gross unrealized losses	(250)
Held-to-maturity, total, estimated fair value	77,575
Held-to-maturity, total, gross unrealized losses	(386)
Available-for-sale, less than 12 months, estimated fair value	694,627
Available-for-sale, less than 12 months, gross unrealized losses	(8,240)
Available-for-sale, 12 months or more, estimated fair value	432,975
Available-for-sale, 12 months or more, gross unrealized losses	(10,094)
Available-for-sale, total, estimated fair value	1,127,602
Available-for-sale, total, gross unrealized losses	(18,334)
US Government Agencies Debt Securities [Member]
Held-to-maturity, less than 12 months, estimated fair value	12,947
Held-to-maturity, less than 12 months, gross unrealized losses	(53)
Held-to-maturity, 12 months or more, estimated fair value	39,913
Held-to-maturity, 12 months or more, gross unrealized losses	(87)
Held-to-maturity, total, estimated fair value	52,860
Held-to-maturity, total, gross unrealized losses	(140)
Available-for-sale, less than 12 months, estimated fair value	66,254
Available-for-sale, less than 12 months, gross unrealized losses	(750)
Available-for-sale, 12 months or more, estimated fair value	82,211
Available-for-sale, 12 months or more, gross unrealized losses	(1,307)
Available-for-sale, total, estimated fair value	148,465
Available-for-sale, total, gross unrealized losses	(2,057)
Collateralized Mortgage Backed Securities [Member]
Held-to-maturity, less than 12 months, estimated fair value	4,094
Held-to-maturity, less than 12 months, gross unrealized losses	(42)
Held-to-maturity, 12 months or more, estimated fair value	6,687
Held-to-maturity, 12 months or more, gross unrealized losses	(149)
Held-to-maturity, total, estimated fair value	10,781
Held-to-maturity, total, gross unrealized losses	(191)
Available-for-sale, less than 12 months, estimated fair value	626,560
Available-for-sale, less than 12 months, gross unrealized losses	(7,458)
Available-for-sale, 12 months or more, estimated fair value	273,883
Available-for-sale, 12 months or more, gross unrealized losses	(6,075)
Available-for-sale, total, estimated fair value	900,443
Available-for-sale, total, gross unrealized losses	(13,533)
US States and Political Subdivisions Debt Securities [Member]
Held-to-maturity, less than 12 months, estimated fair value	11,300
Held-to-maturity, less than 12 months, gross unrealized losses	(41)
Held-to-maturity, 12 months or more, estimated fair value	2,634
Held-to-maturity, 12 months or more, gross unrealized losses	(14)
Held-to-maturity, total, estimated fair value	13,934
Held-to-maturity, total, gross unrealized losses	(55)
Available-for-sale, less than 12 months, estimated fair value	1,813
Available-for-sale, less than 12 months, gross unrealized losses	(32)
Available-for-sale, 12 months or more, estimated fair value	76,781
Available-for-sale, 12 months or more, gross unrealized losses	(2,712)
Available-for-sale, total, estimated fair value	78,594
Available-for-sale, total, gross unrealized losses	(2,744)
Other Securities [Member]
Available-for-sale, less than 12 months, estimated fair value
Available-for-sale, less than 12 months, gross unrealized losses
Available-for-sale, 12 months or more, estimated fair value	100
Available-for-sale, 12 months or more, gross unrealized losses
Available-for-sale, total, estimated fair value	100
Available-for-sale, total, gross unrealized losses