Note 3 - Investment Securities - Securities With Unrealized Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	$ 57,731	$ 149,666
Held-to-maturity, less than 12 months, gross unrealized losses	251	1,187
Held-to-maturity, 12 months or more, estimated fair value	43,611	21,754
Held-to-maturity, 12 months or more, gross unrealized losses	486	86
Held-to-maturity, total, estimated fair value	101,342	171,420
Held-to-maturity, total, gross unrealized losses	737	1,273
Available-for-Sale
Available-for-sale, less than 12 months, estimated fair value	627,807	1,054,739
Available-for-sale, less than 12 months, gross unrealized losses	5,639	25,409
Available-for-sale, 12 months or more, estimated fair value	710,574
Available-for-sale, 12 months or more, gross unrealized losses	19,268
Available-for-sale, total, estimated fair value	1,338,381	1,054,739
Available-for-sale, total, gross unrealized losses	24,907	25,409
US Government Agencies Debt Securities [Member]
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	11,961	32,859
Held-to-maturity, less than 12 months, gross unrealized losses	6	141
Held-to-maturity, 12 months or more, estimated fair value	32,778	19,959
Held-to-maturity, 12 months or more, gross unrealized losses	222	41
Held-to-maturity, total, estimated fair value	44,739	52,818
Held-to-maturity, total, gross unrealized losses	228	182
Available-for-Sale
Available-for-sale, less than 12 months, estimated fair value	91,988	126,325
Available-for-sale, less than 12 months, gross unrealized losses	921	2,301
Available-for-sale, 12 months or more, estimated fair value	25,742
Available-for-sale, 12 months or more, gross unrealized losses	1,030
Available-for-sale, total, estimated fair value	117,730	126,325
Available-for-sale, total, gross unrealized losses	1,951	2,301
Collateralized Mortgage Backed Securities [Member]
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	5,471	11,833
Held-to-maturity, less than 12 months, gross unrealized losses	47	205
Held-to-maturity, 12 months or more, estimated fair value	8,946	1,465
Held-to-maturity, 12 months or more, gross unrealized losses	240	44
Held-to-maturity, total, estimated fair value	14,417	13,298
Held-to-maturity, total, gross unrealized losses	287	249
Available-for-Sale
Available-for-sale, less than 12 months, estimated fair value	510,770	833,393
Available-for-sale, less than 12 months, gross unrealized losses	4,516	17,637
Available-for-sale, 12 months or more, estimated fair value	609,329
Available-for-sale, 12 months or more, gross unrealized losses	16,430
Available-for-sale, total, estimated fair value	1,120,099	833,393
Available-for-sale, total, gross unrealized losses	20,946	17,637
US States and Political Subdivisions Debt Securities [Member]
Held-to-Maturity
Held-to-maturity, less than 12 months, estimated fair value	40,299	104,974
Held-to-maturity, less than 12 months, gross unrealized losses	198	841
Held-to-maturity, 12 months or more, estimated fair value	1,887	330
Held-to-maturity, 12 months or more, gross unrealized losses	24	1
Held-to-maturity, total, estimated fair value	42,186	105,304
Held-to-maturity, total, gross unrealized losses	222	842
Available-for-Sale
Available-for-sale, less than 12 months, estimated fair value	25,049	94,922
Available-for-sale, less than 12 months, gross unrealized losses	202	5,469
Available-for-sale, 12 months or more, estimated fair value	75,404
Available-for-sale, 12 months or more, gross unrealized losses	1,807
Available-for-sale, total, estimated fair value	100,453	94,922
Available-for-sale, total, gross unrealized losses	2,009	5,469
Other Securities [Member]
Available-for-Sale
Available-for-sale, less than 12 months, estimated fair value		99
Available-for-sale, less than 12 months, gross unrealized losses		2
Available-for-sale, 12 months or more, estimated fair value	99
Available-for-sale, 12 months or more, gross unrealized losses	1
Available-for-sale, total, estimated fair value	99	99
Available-for-sale, total, gross unrealized losses	$ 1	$ 2