Note 3 - Investment Securities - Securities with Unrealized Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Held-to-maturity, less than 12 months, estimated fair value	$ 149,666	$ 143,582
Held-to-maturity, less than 12 months, gross unrealized losses	1,187	761
Held-to-maturity, 12 months or more, estimated fair value	21,754	91,256
Held-to-maturity, 12 months or more, gross unrealized losses	86	1,047
Held-to-maturity, total, estimated fair value	171,420	234,838
Held-to-maturity, total, gross unrealized losses	1,273	1,808
Available-for-sale, less than 12 months, estimated fair value	1,054,739	590,993
Available-for-sale, less than 12 months, gross unrealized losses	25,409	4,590
Available-for-sale, 12 months or more, estimated fair value		46,533
Available-for-sale, 12 months or more, gross unrealized losses		582
Available-for-sale, total, estimated fair value	1,054,739	637,526
Available-for-sale, total, gross unrealized losses	25,409	5,172
US Government Agencies Debt Securities [Member]
Held-to-maturity, less than 12 months, estimated fair value	32,859	107,758
Held-to-maturity, less than 12 months, gross unrealized losses	141	567
Held-to-maturity, 12 months or more, estimated fair value	19,959	79,166
Held-to-maturity, 12 months or more, gross unrealized losses	41	828
Held-to-maturity, total, estimated fair value	52,818	186,924
Held-to-maturity, total, gross unrealized losses	182	1,395
Available-for-sale, less than 12 months, estimated fair value	126,325	62,473
Available-for-sale, less than 12 months, gross unrealized losses	2,301	317
Available-for-sale, 12 months or more, estimated fair value		46,023
Available-for-sale, 12 months or more, gross unrealized losses		581
Available-for-sale, total, estimated fair value	126,325	108,496
Available-for-sale, total, gross unrealized losses	2,301	898
Collateralized Mortgage Backed Securities [Member]
Held-to-maturity, less than 12 months, estimated fair value	11,833	7,958
Held-to-maturity, less than 12 months, gross unrealized losses	205	94
Held-to-maturity, 12 months or more, estimated fair value	1,465	8,144
Held-to-maturity, 12 months or more, gross unrealized losses	44	196
Held-to-maturity, total, estimated fair value	13,298	16,102
Held-to-maturity, total, gross unrealized losses	249	290
Available-for-sale, less than 12 months, estimated fair value	833,393	523,185
Available-for-sale, less than 12 months, gross unrealized losses	17,637	4,130
Available-for-sale, 12 months or more, estimated fair value
Available-for-sale, 12 months or more, gross unrealized losses
Available-for-sale, total, estimated fair value	833,393	523,185
Available-for-sale, total, gross unrealized losses	17,637	4,130
US States and Political Subdivisions Debt Securities [Member]
Held-to-maturity, less than 12 months, estimated fair value	104,974	27,866
Held-to-maturity, less than 12 months, gross unrealized losses	841	100
Held-to-maturity, 12 months or more, estimated fair value	330	3,946
Held-to-maturity, 12 months or more, gross unrealized losses	1	23
Held-to-maturity, total, estimated fair value	105,304	31,812
Held-to-maturity, total, gross unrealized losses	842	123
Available-for-sale, less than 12 months, estimated fair value	94,922	281
Available-for-sale, less than 12 months, gross unrealized losses	5,469
Available-for-sale, 12 months or more, estimated fair value
Available-for-sale, 12 months or more, gross unrealized losses
Available-for-sale, total, estimated fair value	94,922	281
Available-for-sale, total, gross unrealized losses	5,469
Other Securities [Member]
Available-for-sale, less than 12 months, estimated fair value	99	1,060
Available-for-sale, less than 12 months, gross unrealized losses	2	137
Available-for-sale, 12 months or more, estimated fair value		510
Available-for-sale, 12 months or more, gross unrealized losses		1
Available-for-sale, total, estimated fair value	99	1,570
Available-for-sale, total, gross unrealized losses	$ 2	138
US Treasury Securities [Member]
Available-for-sale, less than 12 months, estimated fair value		3,994
Available-for-sale, less than 12 months, gross unrealized losses		6
Available-for-sale, 12 months or more, estimated fair value
Available-for-sale, 12 months or more, gross unrealized losses
Available-for-sale, total, estimated fair value		3,994
Available-for-sale, total, gross unrealized losses		$ 6